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THE TRACE EVIDENCE laboratories were on the third floor, and my first stop was the scanning electron microscope, or SEM, which exposed a specimen, such as the metal shaving from the Shephard case, to a beam of electrons. The elemental composition making up the specimen emitted electrons, and images were displayed on a video screen.

In short, the SEM recognized almost all of the one hundred and three elements, whether it was carbon, copper, or zinc, and because of the microscope's depth of focus, high resolution, and high magnification, trace evidence such as gunshot residue or the hairs on a marijuana leaf could be viewed in amazing, if not eerie, detail.

The location of the Zeiss SEM was enthroned within a windowless room of teal and beige wall cupboards and shelves, counter space, and sinks. Because the extremely expensive instrument was very sensitive to mechanical vibration, magnetic fields, and electrical and thermal disturbances, the environment was precisely controlled.

The ventilation and air conditioning system were independent of the rest of the building, and photographically safe lighting was supplied by filament lamps that did not cause electrical interference and were directed up at the ceiling to dimly illuminate the room by reflection. Floors and walls were thick steel-beamed reinforced concrete impervious to human bustling or the traffic of the expressway.

Mary Chan was petite and fair-skinned, a first-rate microscopist, this minute on the phone and surrounded by her complex apparatus. With its instrument panels, power units, electron gun and optical column, X-ray analyzer, and vacuum chamber attached to a cylinder of nitrogen, the SEM looked like a console for the space shuttle. Chan's lab coat was buttoned to her chin, and her friendly gesture told me she would be but a minute.

'Take her temperature again and try the tapioca. If she doesn't keep that down, call me back, okay?' Chan was saying to someone. 'I've got to go now.'

'My daughter,' she said to me as an apology. 'A stomach upset, most likely from too much ice cream last night. She got into the Chunky Monkey when I wasn't looking.'

Her smile was brave but tired, and I suspected she had been up most of the night.

'Man, I love that stuff,' Marino said as he handed her our packaged evidence.

'Another metal shaving,' I explained to her. 'I hate to spring this on you, Mary, but if you could look at it now. It's urgent.'

'Another case or the same one?'

'The fire in Lehigh County, Pennsylvania,' I replied.

'No kidding?' She looked surprised as she slit taped brown paper with a scalpel. 'Lord,' she said, 'that one sounds pretty awful, based on what I heard on the news, anyway. Then the FBI guy, too. Weird, weird, weird.'

She had no reason to know about my relationship with Benton.

'Between those cases and the one in Warrenton, you have to wonder if there isn't some whacko pyro on the loose,' she went on.

'That's what we're trying to find out,' I said.

Chan took the cap off the small metal evidence button and with tweezers removed a layer of snowy cotton, revealing the two tiny bright turnings. She pushed back her roller chair to a counter behind her and proceeded to place a double-sided adhesive square of black carbon tape on a tiny aluminum stub. On this she mounted the shaving that seemed to have the most surface area. It was maybe half the size of a normal eyelash. She turned on a stereo-optical microscope, positioned the sample on the stage, and adjusted the light wand to take a look at a lower magnification before she resorted to the SEM.

'I'm seeing two different surfaces,' she said as she adjusted the focus. 'One real shiny, the other sort of dull gray.'

'That's different from the Warrenton sample,' I said. 'Both surfaces were shiny, right?'

'Correct. My guess would be that one of the surfaces here was exposed to atmospheric oxidation. For whatever reason that might be.'

'Do you mind?' I asked.

She scooted out of the way and I peered through the lenses. At a magnification of four, the metal turning looked like a ribbon of crumpled foil, and I could just barely make out the fine striations left by whatever had been used to shave the metal. Mary took several Polaroid photographs and then rolled her chair back to the SEM console. She pushed a button to vent the chamber, or release the vacuum.

'This will take a few minutes,' she said to us. 'You can wait or go and come back.'

'I'm getting coffee,' said Marino, who had never been a fan of sophisticated technology and most likely wanted to smoke.

Chan opened a valve to fill the chamber with nitrogen to keep contamination, such as moisture, out. Next she pushed a button on the console and placed our sample on an electron optics table.

'Now we got to get it to ten to the minus six millimeters of mercury. That's the vacuum level needed to turn on the beam. Usually takes two or three minutes. But I like to pump it down a little more than that to get a really good vacuum,' she explained, reaching for her coffee. 'I think the news accounts are very confusing,' she then said. 'A lot of innuendo.'

'So what else is new?' I commented wryly.

'Tell me about it. Whenever I read accounts of my court testimony, I always wonder if someone else had been on the stand instead of me. My point is, first they drag Sparkes into it, and to be honest, I was about to think that maybe he had burned his own place and some girl. Probably for money, and to get rid of her because she knew something. Then, lo and behold, there are these two other fires in Pennsylvania, and two more people killed, and there's the suggestion all of it's related? And where's Sparkes been during all this?'

She reached for her coffee.

'Excuse me, Dr Scarpetta. I didn't even ask. Can I get you some?'

'No thank you,' I said.

I watched the green light move across the gauge as the mercury level slowly climbed.

'I also find it odd that this psycho woman escapes from the loony bin in New York - what's her name? Carrie something? And the FBI profiler guy in charge of that investigation suddenly ends up dead. I think we're ready to go,' she said.

She turned on the electron beam and the video display. The magnification was set for five hundred, and she turned it down and we began to get a picture of the filament's current on the screen. At first it looked like a wave, then it began to flatten. She hit more keys, backing off the magnification again, this time to twenty, and we began to get a picture of the signals coming off the sample.

'I'll change the spot size of the beam to get a little more energy.'

She adjusted buttons and dials as she worked.

'Looks like our shaving of metal, almost like a curled ribbon,' she announced.

The topography was simply an enlarged version of what we had seen under the optical microscope moments earlier, and since the picture wasn't terribly bright, this suggested an element with a lower atomic number. She adjusted the scanning speed of the live picture and took away some of the noise, which looked like a snowstorm on the screen.

'Here you can clearly see the shiny versus the gray,' she said.

'And you think that's due to oxidation,' I said, pulling up a chair.

'Well, you've got two surfaces of the same material. I would venture that the shiny side was recently shaved while the other wasn't.'

'Makes sense to me.'

The crinkled metal looked like shrapnel suspended in space.

'We had a case last year,' Chan spoke again as she pressed the frame store button to make photographs for me. 'A guy bludgeoned with a pipe from a machine shop. And tissue from his scalp had a metal filing from a lathe. It was transferred right into the wound. Okay, let's change the back scatter image and see what kind of X-ray we get off that.'

The video screen went gray and digital seconds began to count. Mary worked other buttons on her control panel, and a bright orange spectrum suddenly appeared on the screen against a background of vivid blue. She moved the cursor and expanded what looked like a psychedelic stalagmite.

'Let's see if there are other metals.'

She made more adjustments.

'Nope,' she said. 'It's very clean. Think we got our same suspect again. We'll call up magnesium and see if there's an overlapping of lines.'

She superimposed the spectrum for magnesium over the one for our sample, and they were the same. She called up a table of elements on the video screen, and the square for magnesium was lit up red. We had confirmed our element, and although I had expected the answer we got, I was still stunned by it.

'Do you have any explanation as to why pure magnesium might be transferred to a wound?' I asked Chan as Marino returned.

'Well, I told you my pipe story,' she replied.

'What pipe?' Marino said.

'Only thing I can think of is a metal shop,' Chan went on. 'But I would think that machining magnesium would be unusual. I mean, I can't imagine what for.'

'Thanks, Mary. We've got one more stop to go, but I'm going to need you to let me have the shaving from the Warrenton case so I can take it over to firearms.'

She glanced at her watch as the phone rang again, and I could only imagine the caseload awaiting her.

'Right away,' she said to me generously.

The firearms and toolmarks labs were on the same floor and were really the same section of science, since the lands and grooves and firing pin impressions left on cartridge cases and bullets were, in fact, the toolmarks made by guns. The space in the new building was a stadium compared to the old, and this spoke sadly to the continuing deterioration of the society beyond our doors.

It was not unusual for schoolchildren to hide handguns in their lockers, or show them off in the bathrooms, and carry them on the school bus, it seemed, and it was nothing for violent offenders to be eleven and twelve years old. Guns were still the top choice for killing oneself or one's spouse, or even the neighbor with the constantly barking dog. More frightening were the disgruntled and insane who entered public places and started blasting away, explaining why my office and the lobby were protected by bulletproof glass.

Rich Sinclair's work area was carpeted and well lighted, and overlooked the coliseum, which had always reminded me of a metal mushroom about to take flight. He was using weights to test the trigger pull of a Taurus pistol, and Marino and I walked in to the sound of the hammer clicking against the firing pin. I was not in a chatty mood and did my best not to seem rude as I told Sinclair outright what I needed, and that I needed it now.

'This is the metal turning from Warrenton,' I said, opening that evidence button. 'And this is the one recovered from the body in the Lehigh fire.'

I opened that evidence button next.

'Both have striations that are clearly visible on SEM,' I explained.

The point was to see if the striations, or toolmarks, matched, indicating that the same instrument had been used to produce the magnesium shavings that had been recovered this far. The ribbons of metal were very fragile and thin, and Sinclair used a narrow plastic spatula to pick them up. They weren't very cooperative and tended to jump around as if they were trying to escape as he coaxed them from their sea of cotton. He used squares of black cardboard to center the shaving from Warrenton on one, and the shavings from Lehigh on the other. These he placed on stages of the comparison microscope.

'Oh yeah,' Sinclair said without pause. 'We've got some good stuff.'

He manipulated the shavings with the spatula, flattening them some as he bumped the magnification up to forty.

'Maybe a blade of some type,' he said. 'The striations are probably from the finishing process and end up being a defect because no finishing process is going to be perfectly smooth. I mean, the manufacturer's going to be happy, but he's not at our end seeing this. There, here's an even better area, I think.'

He moved aside so we could take a look. Marino bent over the eyepieces first.

'Looks like ski tracks in snow,' was his comment. 'And that's from the blade, right? Or whatever?'

'Yes, imperfections, or toolmarks, made by whatever shaved this metal. Do you see the match, when one shaving is lined up with the other?'

Marino didn't.

'Here, Doc, you look.' Sinclair got out of my way.

What I saw through the microscope was good enough for court, the striations of the Warrenton shaving in one field of light matching the striations of the shaving in the other. Clearly, the same tool had shaved something made of magnesium in both homicide cases. The question was what this tool might be, and because the shavings were so thin, one had to consider a sharp blade of some type. Sinclair made several Polaroid photographs for me and slid them into glyassine envelopes.

'Okay, now what?' Marino asked as he followed me through the center of the firearms lab, past scientists busy processing bloody clothing under biohazard hoods, and others examining a Phillips screwdriver and machete at a big U-shaped counter.

'Now I go shopping,' I said.

I did not slow down as I talked but, in fact, was getting more frantic because I knew I was getting closer to reconstructing what Carrie or her accomplice or someone had done.

'What do you mean, shopping?'

Through the wall I could hear the muffled bangs of test fires in the range.

'Why don't you check on Lucy?' I said. 'And I'll get back to both of you later.'

'I don't like it when you do this later shit,' Marino said as elevator doors parted. 'That means you're running around on your own and poking your nose in things that maybe you shouldn't. And this ain't the time for you to be out on the street with nobody around. We got not a clue where Carrie is.'

'That's right, we don't,' I said. 'But I'm hoping that's going to change.'

We got out on the first floor, and I headed with purpose to the door leading to the bay, where I unlocked my car. Marino looked so frustrated, I thought he might launch into a tantrum.

'You want to tell me where the hell you're going?' he demanded at the top of his voice.

'A sports store,' I said, cranking the engine. 'The biggest one I can find.'

That turned out to be Jumbo Sports south of the James, very close to the neighborhood where Marino lived, which was the only reason I was aware of the store, since prowling for basketballs, frisbees, free weights, and golf clubs rarely entered my mind.

I took the Powhite Parkway, and two toll booths later was exiting on Midlothian Turnpike, heading toward downtown. The sports store was -big and built of red brick, with stick figures of red-painted athletes framed in white on the outside walls. The parking lot was unexpectedly full for this time of day, and I wondered how many well-toned people spent their lunch hours here.

I had no idea where anything was and had to take a few moments to study the signs above miles of rows. Boxing gloves were on sale, and there were exercise machines capable of tortures I did not know. Racks of clothes for every sport were endless and in blazing colors, and I wondered what had happened to civilized white, which was still what I wore on the much-appreciated occasion I found time to play tennis. I deduced that knives would be with camping and hunting gear, a generous area against the back wall. There were bows and arrows, targets, tents, canoes, mess kits, and camouflage, and at this hour, I was the only woman who seemed interested. At first, no one was inclined to wait on me as I hovered patiently over a showcase of knives.

A sunburnt man was looking for a BB gun for his son's tenth birthday, while an older man in a white suit was inquiring about snakebite kits and mosquito repellent. When my patience was no more, I interrupted.

'Excuse me,' I said.

The clerk, who was college age, didn't seem to hear me at first.

'Thing is, you should check with your doctor before using a snakebite kit,' the clerk was saying to the elderly man in white.

'How the hell am I supposed to do that when I'm out in the middle of the woods somewhere and some copperhead's just bit me?'

'I meant check with him before you go out in the woods, sir.'

As I listened to their backward logic I could stand it no longer.

'Snakebite kits are not only useless, but they're harmful,' I said. 'Tourniquets and local incision, sucking out the venom and all that just make matters worse. If you get bitten,' I said to the man in white, 'what you need to do is immobilize that part of your body, and avoid damaging first aid, and get to a hospital.'

The two men were startled.

'So there's no point in taking anything along?' the man in white asked me. 'No point in buying anything, you're saying?'

'Nothing but a good pair of boots and a walking stick you can poke around with,' I replied. 'Stay out of tall grass and don't stick your hands into hollows or holes. Since venom is transported through the body by the lymphatic system, broad compression bandages - like an Ace bandage - are good, and a splint to keep the limb absolutely immobilized.'

'You some kind of doctor?' the clerk asked.

'I've dealt with snake bites before.'

I didn't add that in those instances, the victims had not lived.

'I'm just wondering if you have knife sharpeners here,' I said to the clerk.

'Kitchen sharpeners or ones for camping?'

'Let's start with camping,' I said.

He pointed to a wall where a vast variety of whetstones and other types of sharpeners hung from pegs. Some were metal, others ceramic. All of the brands were proprietary enough not to reveal the composition on the packages. I scanned some more, my eyes stopping on a small package on the bottom row. Beneath clear plastic was a simple rectangular block of grayish-silver metal. It was called a fire starter and was made of magnesium. Excitement mounted as I read the instructions. To start a fire, one simply needed to scrape a knife on the surface of the magnesium and build a pile of shavings as small as the size of a quarter. Matches weren't necessary, for the fire starter included a sparking insert for ignition.

I hurried back through the store with half a dozen of the magnesium starters in hand, and in my haste got tangled up in one section, then another. I wound through bowling balls and shoes, and baseball gloves, and ended up in swimming, where I was instantly captivated by a display of neon-colored swimming caps. One of them was hot pink. I thought of the residue found in Claire Rawley's hair. I had believed from the start that she had been wearing something on her head when she was murdered, or at least when the fire reached her.

A shower cap had been considered but briefly, for its thin, plastic material wouldn't have lasted five seconds in the heat. What had never entered my mind was a swimming cap, and as I quickly riffled through racks of them, I discovered that all were made of Lycra or latex or silicone.

The pink one was silicone, which I knew would hold up in extreme temperatures far better than the others. I purchased several of them. I drove back to my office and was lucky I didn't get a ticket because I was passing people, no matter the lane. Images seized my mind, and they were too painful and horrific to entertain. This was one time I hoped my theory was wrong. I was speeding back to the labs because I had to know.

'Oh Benton,' I muttered as if he were near me. 'Please don't let this be so.'

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