He arrived at the Firearm Analysis Unit at 11 A.M., two still-warm submarine sandwiches from Giamela’s in a bag with sauce on the side. They went into a break room to eat, and Pistol Pete moaned after taking his first bite of meatball sub—so loudly that he drew two other firearm analysts to the room to see what was going on. Sargent and Bosch grudgingly shared their sandwiches with them, Bosch making friends for life.

When they got to Sargent’s worktable, Bosch saw that the Beretta he had brought in was already held in a vise with the left side angled up. The frame had already been polished smooth with steel wool in preparation for Sargent’s effort to raise the serial number.

“We’re ready to go,” Sargent said.

He pulled on a pair of heavy rubber gloves and a plastic eye shield and took his place on the stool in front of the vise. He then pulled the mounted magnifying glass over by its arm and snapped on the light.

Bosch knew that every gun legally manufactured in the world carried a unique serial number through which ownership as well as theft could be traced. People who wanted to hinder the tracing of a gun often filed the serial number off with a variety of tools or attempted to burn it off with acids.

But the manufacturing of the weapon and the stamping procedure involved in placing the serial number on it in the first place gave law enforcement a better-than-good chance of recovering the number. When a serial number is stamped on a gun’s surface during manufacture, the procedure compresses the metal below the letters and numbers. The surface may later be filed or acid burned, but it very often still leaves the compression pattern beneath. Various methods can be used to draw the serial number out. One involves the application of a mixture of acids and copper salts that reacts to the compressed metal, revealing the numbers. Another involves the use of magnets and iron residue.

“I want to start with Magnaflux because if it works it’s quicker and it doesn’t damage the weapon,” Sargent said. “We still have ballistics work to do with this baby and I want to keep it in working order.”

“You’re the boss,” Bosch said. “And as far as I’m concerned, quicker is better.”

“Well, let’s see what we get.”

Sargent attached a large, round magnet on the underside of the gun, directly below the slide.

“First we magnetize . . .”

He then reached up to a shelf over the table and took down a plastic spray bottle. He shook it and then pointed it at the weapon.

“Now we go with Pistol Pete’s patented iron-and-oil recipe . . .”

Bosch leaned in close as Sargent sprayed the gun.

“Iron and oil?”