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explainer: Answers to your questions about the news.

The Secrets Police KeepHow cops decide which details of a crime to make public.

By Doree Shafrir
Posted Thursday, Aug. 17, 2006, at 7:10 PM ET

Download the MP3 audio version of this story here, or sign up for The Explainer's free daily podcast on iTunes.

According to some news reports, the man arrested for killing JonBenet Ramsey knew details of the case that had not been publicly released. In high-profile cases, police often keep some details about the crime scene secret as they pursue their investigation. How do the police decide which facts to withhold from the press?

They generally keep quiet about a few details that only the killer could know—anything from an object left at the scene to the way a garrote was tied around a victim's neck. The chief detective on a case usually decides what evidence to make public and what to withhold, although his choices are sometimes influenced (or overruled) by the police chief. In a high-profile case, the police department will generally hold a press conference immediately after discovering a crime but release few details about the case. Later, once those assigned to the case have had a chance to sift through the evidence, police will decide which details to make public.

Police withhold evidence to help determine whether suspects they catch are guilty. If a suspect possesses nonpublic information about a crime, police reason, it is likely that he either committed it or knows who did. And a suspect's knowledge of nonpublic information can be used against him in court. Of course, it's tricky to get a suspect to reveal such information. During interrogations, police officers refrain from discussing nonpublic details of a crime in the hopes that a suspect will slip up and reveal that he knows some of them.

Withholding evidence also helps police rule out the possibility that a suspect might be confessing to a crime he didn't commit. False confessors often piece together information about a crime by obsessively tracking news stories and assembling as many details as they can. When questioned by police, they can sometimes offer convincing explanations of how they committed the crime. Police officers are trained to quickly determine whether such suspects know about the evidence they've withheld.

There's no surefire way to ensure that details about a case don't leak out during an investigation, but police generally try to limit the number of people working on a case in order to minimize the risk of leaks.

Explainer thanks former police detective Lee Lofland, author of The Book of Police Procedures and Investigations.