Could your phone testify against you? Prosecutors are increasingly using cellphone data in court, which raises constitutional issues that pit public safety against privacy rights.

• Author: Barnard, Anne
• Position: NATIONAL

Mikhail Mallayev pleaded not guilty when he was charged in the contract killing of a New York orthodontist whose wife wanted him dead. While he stayed off his cellphone the morning of the shooting, he chatted away later, unaware that his phone was acting like a tracking device and would destroy his alibi--that he was not in New York the day of the killing--and lead to his conviction.

In another case, Darryl Littlejohn, a New York nightclub bouncer, made call after call on his cellphone as he drove from his home to a desolate part of Brooklyn to dump the body of the graduate student he was convicted last summer of murdering.

The pivotal role that cellphone records played in these two murder trials highlights the growing use in law enforcement of increasingly sophisticated cellular tracking techniques to keep tabs on suspects before they are arrested, and build criminal cases against them by mapping their past movements.

But cellphone tracking is raising concerns about civil liberties in a debate that pits public safety against privacy rights. Existing laws do not provide clear guidelines: Federal wiretap laws, outpaced by technological advances, do not explicitly cover the use of cellphone data to pinpoint a person's location, and local court rulings vary widely across the country.

In one case that unsettled cellphone companies, a sheriff in Alabama told a carrier he needed to track a cellphone in an emergency involving a child--she turned out to be his teenage daughter, who was late coming home from a date.

Cellphone tracking isn't the only way digital technology is challenging the legal system. More mistrials are being declared because jurors use their cellphones to do their own Internet research, upending centuries of legal procedure in which the judge decides what information the jury can hear.

Courts are also struggling with how to handle search warrants when they apply to digital information. The Fourth Amendment, which protects against "unreasonable searches and seizures," requires investigators to show probable cause and specify where they want to search and what they're looking for in order to get a warrant. While searching, if they find evidence of a different crime, they're allowed to act on that unexpected evidence if it's in "plain view."

The question now is, how does the concept of "plain view" apply to unexpected finds in computer files and cellphones?

THE IMPACT OF G.P.S.

The ability to track suspects' movements through their...