When 24-year-old La'Toyia Figueroa disappeared on July 18, 2005, Philadelphia police quickly tapped into a trove of digital clues: the cell-phone records of the missing woman and her ex-boyfriend, Stephen Poaches.

By learning whom they had called and which towers had transmitted the signals, investigators were able to retrace their movements through the city, from her prenatal appointment at Pennsylvania Hospital, to lunch at a seafood take-out restaurant, to his apartment in West Philadelphia.

There, at 2:42 p.m., the calls to and from Figueroa's phone stopped.

A month after she vanished, police arrested Poaches, father of her unborn child, as he was moving her body from the weedy lot in Chester where he had dumped it.

For Poaches' double-murder conviction in October 2006, "the phone records were key," said Special Agent William Shute of the FBI's Philadelphia office, who testified as an expert witness on the cellular data.

About 250 million people in the United States are cell-phone subscribers. Of those, an inestimable number are up to no good. But even as criminals use cellular technology to hatch plots, keep tabs on drug buys, and trade child pornography, police increasingly are using the lawbreakers' phones to solve cases and get convictions.

Consider what savvy investigators can mine: address books, call histories, pictures, ringtones, voice mail, text messages, to-do lists and appointments. Even deleted data often can be salvaged with specialty software.

"It's phenomenal," Steve D'Aguanno, an assistant U.S. attorney in Camden, said of the technology. And thanks to the CSI programs and their TV ilk, "it is the type of evidence juries are now coming to expect."

This spring, officers from police departments in the Pennsylvania suburbs, Philadelphia and South Jersey have begun learning to unlock at least some cell secrets from Randy Becker, a former Oregon State Police officer who conducts classes for BKForensics in Warrington, Bucks County. The firm also manufactures hardware to download cell data and, in tough cases, provides technical assistance to law enforcement.

"The criminal justice system is always a step behind criminals," Becker said.

Retrieving evidence from computers has become routine, but, he said, "analysis of cell phones is much more difficult" because so many operating systems are on the market. Different brands or models require specific hardware, not only complicating forensics but also making it prohibitively expensive for small police departments.

That's where Roy Calarese and Joe Walton come in.

The small room where the two Chester County police detectives work looks more like an electronics repair store than a high-tech forensics lab. With so rare a specialty, the two take on cases from jurisdictions that cannot afford a computer crimes unit.

After about 50 cell-phone-related cases in the last three years, Calarese and Walton know what's happening under the hood. In less than a minute, with a turn of a screw, they expose the phone's innards to look for a SIM card, or memory chip, which holds the user's information and allows the phone to connect to the network.

Not all phones have the card, Walton said, but when one is found, the "data dump" yields a series of binary numbers - 0s and 1s - on the computer screen.

"If a picture is on a phone, it may have a header," a set of different numbers or letters, Walton explained as he scrolled through the lines.

It might look indecipherable. But, Calarese said, it can "turn a case based solely on a victim's statements into a case with evidence."

However, if improperly retrieved, cellular data is off-limits in a criminal investigation. In order to access it, police first need a federal court order or state search warrant. Without it, the information cannot be used.

Law enforcement is "not just making a conscious decision to invade people's privacy," D'Aguanno said. "We get these records when we know there is criminal activity."

D'Aguanno came to appreciate the impact of cell-phone forensics while prosecuting a 2005 home-invasion case in Gloucester Township.

Three of the four robbers were captured in Kensington after a high-speed chase across the Walt Whitman Bridge ended in a crash. One man escaped, but police identified him as William Hernandez through a bank card left in the wrecked van.

In what is called "historical cell-site analysis," FBI agent Shute was able to use Hernandez's Nextel phone records to place him in Gloucester Township, then on the bridge at the time of the chase, then in the Kensington area just after the crash. "That is where his phone is for the next 45 minutes," said Shute, who mapped the trail on a spreadsheet.

Most of the calls were made to his girlfriend. "We watched her phone come into the area," the agent said, "and disappear."

The four were found guilty and sentenced to 35 to 40 years in prison.

"I don't think he would have been convicted, period," D'Aguanno said about the case against Hernandez. "The phone records really put it over the top."

Because most criminals use cell phones no more judiciously than the law-abiding populace, "we are seeing all kinds of cases solved" from the evidence unwittingly left behind, said Cpl. Jeff Whitmarsh of the Delaware State Police.

When 11-month-old Michael DiGirolamo was found wandering in the parking lot of Christiana Hospital in Newark, Del., police were able to use a cell-phone call to implicate Rosario DiGirolamo, 33, of Millstone, in the toddler's abandonment and, eventually, the murder of the boy's mother.

They found that the pings - signals given off when a phone with a Global Positioning System receiver is used - were from a tower adjacent to the hospital.

"It was a key piece of evidence that broke this case wide open for us," Whitmarsh said.

Cell-phone forensics has piqued the interest of not only law enforcement. Along with police, Becker's training classes draw corporate security officials who want to access data from company cell phones to make sure employees are not talking to competitors or taking pictures of trade secrets.

"E-discovery is getting big," Becker said.

Offenders might not wind up in prison. But "depending on the work policies," he said, "it could lead to job loss."