'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.