Still, the authority granted to a detective on that small parcel of land where a body happens to fall speaks to the importance and fragility of a crime scene. Homicide men are fond of reminding one another-and anyone else who will listen-that a detective gets only one chance at a scene. You do what you do, and then the yellow plastic police-line-do-not-cross strips come down. The fire department turns a hose on the bloodstains; the lab techs move on to the next call; the neighborhood reclaims another patch of pavement.

The crime scene provides the greater share of physical evidence, the first part of a detective’s Holy Trinity, which states that three things solve crimes:

Physical evidence.

Witnesses.

Confessions.

Without one of the first two elements, there is little chance that a detective will find a suspect capable of providing the third. A murder investigation, after all, is an endeavor limited by the very fact that the victim-unlike those who are robbed, raped or seriously assaulted-is no longer in a position to provide much information.

The detective’s trinity ignores motivation, which matters little to most investigations. The best work of Dashiell Hammett and Agatha Christie argues that to track a murderer, the motive must first be established; in Baltimore, if not on the Orient Express, a known motive can be interesting, even helpful, yet it is often beside the point. Fuck the why, a detective will tell you; find out the how, and nine times out of ten it’ll give you the who.

It’s a truth that goes against the accepted grain and court juries always have a hard time when a detective takes the stand and declares he has no idea why Tater shot Pee Wee in the back five times, and frankly, he could care less. Pee Wee isn’t around to discuss it, and our man Tater doesn’t want to say. But, hey, here’s the gun and the bullets and the ballistics report and two reluctant witnesses who saw Tater pull the trigger and then picked the ignorant, murdering bastard from a photo array. So what the hell else you want me to do, interview the goddamn butler?

Physical evidence. Witnesses. Confessions.

Physical evidence can be anything from a usable latent print on a water glass to a spent bullet pulled from the drywall. It can be something as obvious as the fact that a house has been ransacked, something as subtle as a number on the victim’s telephone pager. It can be the victim’s clothes, or the victim himself, when the small, dark specks of stipplin against fabric or skin show that the wound was inflicted at close range. Or a blood trail that shows the victim was attacked first in the bathroom, then pursued into the bedroom. Or the what’s-wrong-with-this-picture game, in which a witness is claiming that no one else was home, but there are four used plates on the kitchen counter. Physical evidence from a crime scene can also be measured by what is not present: the absence of any forced entry to a house; the lack of blood from a gaping neck wound, suggesting that the victim was killed elsewhere; a dead man in an alley with the trouser pockets pulled inside-out, indicating that robbery was the motive.

There are, of course, those sacred occasions when physical evidence itself identifies a suspect. A spent bullet is recovered intact and with little apparent mutilation, so that it can be matched ballistically against a recovered weapon or against same-caliber projectiles from another shooting in which a suspect has been identified; a semen sample recovered in a vaginal swab is DNA-matched to a possible assailant; a footprint found near a body in the dirt of a railroad bed is paired with a sneaker worn by a suspect into the interrogation room. Such moments offer clear evidence that the Creator has not yet shelved his master plan and that, for one fleeting moment, a homicide detective is being used as an instrument of divine will.

More often, however, the physical evidence gathered at the crime scene provides the detective with information that is less absolute, but nonetheless essential. Even if the evidence doesn’t lead directly to a suspect, the raw facts provide a rough outline of the crime itself. The more information that a detective brings away from the scene, the more he knows what is possible and what is not. And in the interrogation rooms, that counts for a great deal.

In the soundproof cubicles used by the homicide unit, a witness will readily claim he was asleep in bed when the shooting started in the next room, and he will maintain the deceit up to the point when a detective confronts him with the fact that the sheets were not disturbed. He will tell the detectives that the shooting could not have been over drugs, that he knows nothing about drugs, until the detective tells him they’ve already found 150 caps of heroin under his mattress. He will claim that only the lone assailant was armed and there was no shootout until the detective makes it clear that.32 and 9mm casings were both recovered in the living room.

Denied the knowledge provided by physical evidence, a detective walks into the interrogation room without leverage, without any tool to pry truth from suspects or reluctant witnesses. The bastards can lie themselves blind and the detectives, disbelieving and frustrated, can scream at them for lying themselves blind. Without physical evidence, there is only stalemate.

Beyond those who don’t want to talk, the physical evidence keeps honest those who willingly volunteer information. Seeking to cut deals on their own charges, inmates at the city jail routinely claim to have heard fellow prisoners boast about or confess to murders, but detectives seriously pursue only those statements that include details from the crime scene that only a perpetrator could know. Likewise, a confession obtained from a suspect that includes details of the crime known only to the killer is inherently more believable in court. For these reasons, a detective returns from every crime scene with a mental list of essential details that he plans to withhold from newspaper and television reporters who will be calling the homicide office half an hour after the body hits the ground. Typically, a detective will hold back the caliber of the weapon used, or the exact location of the wounds, or the presence of an unusual object at the scene. If the murder occurred inside a house rather than on a street where a crowd can gather, the investigator may try to withhold a description of the clothes worn by the victim or the exact location of the victim’s body in the house. In the Latonya Wallace case, Landsman and Pellegrini were careful not to mention the ligature marks on the victim’s neck or that a cord or rope was used in the strangulation. They also kept the evidence of sexual molestation, or at least they tried to keep it-a week after the murder, a colonel felt the need to reveal the motive for the slaying to concerned parents at a Reservoir Hill community meeting.

From a detective’s point of view, no crime scene is better than a body in a house. Not only does a murder indoors mean that details can be kept from gathering crowds or prying reporters, but the house itself offers immediate questions. Who owns or rents the house? Who’s living there? Who was inside at the time? Why is my victim inside this house? Does he live here? Who brought him here? Who was he visiting? And call for a wagon, because everyone in the place is going downtown.

To murder someone in a house, a killer has first got to gain entry, either at the invitation of the victim or by forcing a door or window. Either way, something is gained by the investigator. The absence of forced entry suggests that the victim and assailant were probably known to each other; forced entry allows for the possibility that the killer has left fingerprints on a windowpane or door frame. Once inside a house, the killer may well touch a variety of utensils and smooth surfaces, leaving more latent prints. If the killer sprays some bullets around, most of the stray shots will appear as holes in the walls, in the ceiling, in the furniture. If the victim struggles and the assailant is injured, blood spatter or pulled hairs will be more easily discovered in the limited confines of a living room. The same thinking applies to loose fibers and other trace evidence. A lab tech can take a vacuum to a three-bedroom house in under an hour, then turn the vacuum bag’s contents over to the whitecoats for sifting in the fifth-floor labs.