Her audience nodded.

“But in this case, the rookie patrolman, Justin, saved me and my team a great deal of grief. He peered inside the motel room, noted the carnage, and, not being an idiot, called off the ambulance, as it wasn’t going to matter. Justin secured the scene, then notified dispatch to contact my unit instead. Job well done.

“Boston homicide works as three-people squads. I’m a sergeant detective, meaning I’m the so-called leader of my squad, though my teammates, Phil and Neil, would love to argue. Each squad takes a turn being on call. That Saturday night, we were the lucky squad to be summoned at two A.M. to a sleazy motel in downtown Boston where the rooms rent by the hour.

“Now, policing is a matter of playing the odds. Hourly-rate hotel in that area of Boston, I’m already thinking drug overdose, or maybe pimp versus hooker or dealer versus dealer. These things happen. So I wasn’t totally shocked to walk into a bloody hotel room and discover the body of a naked man on the floor. What caught me off guard was that the body had been partially dismembered—the right leg hacked off just above the knee.

“Which had then been left behind in the room’s bathtub.”

• • •

“All right.” D.D. made her voice brisk. “You guys are the aspiring crime aficionados. Picture yourself as a homicide detective walking into a crime scene. What’s the first thing you do?”

Hands shot in the air. She went with a middle-aged woman in the back, who stated immediately: “Secure the perimeter.”

“Partial credit,” D.D. granted. “In theory, the first responder establishes the perimeter. But screw the perimeter, screw the investigation, which is why I feel a need to at least check things out when I first appear. In this case, our rookie officer Justin had been paying attention during training. He hadn’t just cordoned off the room, but most of the parking lot. Aggressive? Maybe. But our DG—dead guy—had to get into the motel room somehow, right? So best to protect all entrances and exits from contamination. Next?”

“Establish a murder log,” someone called out.

“Wow, you guys do watch a lot of TV. Partial credit again. Generally, a uniformed patrol officer or district detective will take this job, stationing themselves just inside of the crime scene tape and recording every person who crosses the line into the so-called murder book. Basically, upon arrival, every working officer, myself included, must supply a badge number for the log. You know why?”

“Locard’s principle.”

D.D. squinted the respondent in the front row. Attractive male, nice jacket, short wavy brown hair. She thought she recognized him as one of the speakers, Joe Finder, known for his clever thrillers.

“Show-off,” she informed him. He grinned.

“Joe, here, is being technical about things. But yes, Locard’s principle holds that everyone who enters a crime scene will transfer something to that scene—hair, fiber, fingerprints, you name it. It’s the basic tenet behind forensic science: By working the scene, forensic experts can identify these transfer elements and then smart detectives like me can use that information to catch the bad guy. Of course, that same principle applies to even the good guys. Sure, we don gloves and, in the case of a really bloody scene, maybe hair caps and foot booties. But that doesn’t mean we won’t inevitably leave something behind. So the murder log has two functions. One, to help forensics sort out our footprints, maybe even a really stupid officer’s fingerprint, from the relevant evidence gathered at the scene. Also, for legal reasons, you need a record of everyone at the scene. Now, here’s a question, the rookie officer, having answered his first homicide call, having done a great job of setting up his first perimeter, then snaps a photo to show off to his buddies. One of the perks of the job, or do I now seize his phone?”