“Show me what?”

“Could you come down to the lab?”

I sigh, rolling my eyes for Charlotte’s benefit. She rolls hers back for mine.

“Half an hour,” I say to Castro.

“Excellent,” he replies. I imagine him on the other end of the line, pumping his fist in triumph. Whatever has got him so worked up, it better have the same effect on me.

CHAPTER 19

The moment the gun is in his hand, Edgar Castro’s eyes light up. He uses a serrated folding knife to remove the plastic tie running through the barrel, then eyeballs the breech to make sure there’s not a round in the chamber. When he passes it across the desk, I can sense his reluctance to let go. After double-checking for safety, I release the slide. It slams shut with a familiar metallic snap.

“Everything look right to you?” he asks.

I give the pistol a closer inspection. The blued finish is worn down on the edges, probably from holster wear, and the plastic factory grips have been replaced by checkered cocobolo. Along the front strap, a strip of skateboard tape provides tacky traction. The barrel is stamped.40 S &W, the cartridge our service pistols are chambered for. Thanks to my time clerking in the gun shop as a young man, I have an abiding awe for the trusty.45, but over the years I’ve come to respect the smaller, hotter.40. Apart from the fancy hardwood, this gun is a tool, plain and simple, the same as the one I carry every day.

“The tape’s a little ghetto,” I say, “but otherwise it looks fine.”

Castro’s little corner of csu is dark, packed with computer screens, lit by arc lamps, littered with a recycler’s dream supply of empty Dr. Pepper cans. The workspace, wiped clean apart from the pile of plastic evidence bags, was created by fitting a tabletop over a shoulder-to-shoulder rank of filing cabinets. He scrounged a desk from somewhere, too, a castoff from the dark days before the current cubicle system was installed.

He fishes a loaded magazine from a separate evidence bag, sliding it over.

“This stuff ’s all been checked for prints already?”

He nods. “It’s cool. Now, does that look right?”

The magazine’s weight feels good in my hand. I press down on the uppermost cartridge with my thumb, testing the spring’s resistance.

“I don’t get what you’re asking,” I say. “If there’s something wrong here, you’re going to have to point it out.”

He takes pistol and magazine back, inserts one into the other, then works the slide.

“Is that really necessary?” Even though the muzzle is safely aimed at the ground, I wince a little. Castro doesn’t inspire gun-handling confidence. He seems just the sort for an accidental discharge. Fortunately, the other technicians seem to give him a wide berth. The only other occupant of this particular room – calling it a lab would only dignify what looks like an oversized storage closet for high-tech equipment – vacated as soon as I showed up.

He drops the magazine and ejects the chambered round. It flips through the air, thumping to rest on the gray carpet.

“Just what you’d expect, right?” he says, setting the weapon down between us. “Now take a look at this.”

The next item on his show-and-tell list is a double magazine carrier, nice tan leather from Milt Sparks, the sort of thing you clip to your belt to keep spare ammunition handy. Thomson seems to have been a man after my own heart, judging from the grips and gun leather, splashing out for the good stuff.

“Nice,” I say. “So what?”

“Look at the magazines.”

With a sigh I withdraw the mags. They look the same as the other one. I thumb down the top round again, letting it spring back. Then the difference registers. The shape of the cartridges. Instead of the long, flat plane of a.40 caliber round, these are bottlenecked at the point where the bullet fits into the brass to accommodate a smaller projectile. I slide a round out, inspecting the bottom.

“These are.357 sig,” I say.

“Exactly. And they’re both the same. Now, the spent brass recovered inside Thomson’s vehicle was.40 caliber, and so are the rounds in the clip we found inside the gun. We dug the bullet out of the door pillar, and it’s.40 caliber, too, and a match for the barrel. So the fatal round was fired through that barrel, from that magazine. Everything is how it should be.”

“Except this.” I tap the.357 sig round against the desk.

“Right. So my question is, why was Thomson carrying one kind of ammo on his belt and another kind in his gun? You can load.357s into the same magazine as a.40, but have you ever tried firing one from a.40 caliber pistol? A little hint: don’t even try it. So either this guy Thomson was monumentally brain dead – I mean, really – or he didn’t pay much attention to detail. Or…”

“Or what?” I ask, though I already know where he’s going.

Castro picks up the gun and fieldstrips it, removing the entire slide assembly, then pulling the barrel out. He holds it up to his eye like a telescope.

“This is the barrel that fired the bullet that killed Detective Thomson,” he says, “but does that mean this is the gun?” He taps the pile of disassembled metal. “Not necessarily. You know what happened when the.357 sig round first came out? A lot of guys believed the hype, so they went out and bought drop-in barrels to convert their.40s into.357s. It’s as simple as fieldstripping the piece and putting a different barrel inside. Like this.” He opens a drawer, removing a silvered drop-in barrel, which he fits into the slide assembly, putting the pistol back together. When he’s done, he racks the slide a few times. “Now, if you stick one of Thomson’s spare mags in, this baby’s good to go. That’s really all it takes.”

He blossoms his hands like a magician, then sits back looking very satisfied. The theory forming in my head goes something like this. Someone in the passenger seat pulls a gun, pressing it against Thomson’s head. Pulling the trigger creates a contact wound, but to make the ballistics look right, the killer has to improvise. And it could only be improvisation. If he’d planned ahead, he might have found a way to cover his tracks better, but in the moment he just has to make do. He’s shot Thomson – now what? So he switches barrels between his own pistol and Thomson’s. Meaning the killer was armed with a sig Sauer P229, as well. One of us, more than likely.

“So let me get this straight,” I say. “You think someone shot Thomson, then fieldstripped both the murder weapon and Thomson’s gun, swapping the barrels and the magazines? Only why would he switch mags?”

Castro shrugs. “Maybe he’s doing it, and he realizes there’s a difference in the bullets. He wouldn’t be able to chamber a.357 round in a.40 caliber barrel, so that might be what tipped him off. He tries it, gets a jam, and has to switch the mags.”

“Then why leave Thomson’s spares behind?”

“He doesn’t see them,” he says. “Detective Thomson’s sitting down. He was wearing the mag carrier behind his left hip, so they’d be on the opposite side.” He pats his left hip. “Plus everything’s happening so fast. The perp messed up, basically.”

“Any prints on the barrel? What about the rounds in the magazine?”

He shakes his head. “There he didn’t mess up. They were wiped clean. Which is strange when you think about it. Those rounds should have prints all over them, right?”

“Mine would.”

I go through the motions in my mind, stripping the guns, changing the barrels, sorting out the ammunition. It’s complicated, but under stress someone familiar both with handguns and forensics could make it happen. And missing the extra magazines would be an easy mistake. The scene would have been dark, he’d be pumping with adrenaline, a dead man on the seat next to him, the rain hammering on the roof. There’s a problem, though.

“Maybe Thomson was absentminded. Maybe he forgot he’d loaded those mags with.357 sig rounds.”