It was a crucial moment in 2007 during the trial of Paul Cortez, an actor and yoga teacher who was ultimately convicted of killing his former girlfriend Catherine Woods, a dancer who was working as a stripper.
After weeks of testimony and a parade of witnesses, the case against Mr. Cortez boiled down to this: a bloody fingerprint and data collected from a cell phone.
A record from a T-Mobile cell phone transmission tower on the day Ms. Woods was murdered showed that Mr. Cortez called her 13 times in the hour and a half before her death, and then never again. He had told the police in a written statement that he made the calls from his home.
But as he called, the record showed his cell signal hitting a tower near his apartment, and gradually shifting to towers near Ms. Woods’s apartment. At trial, when the prosecutor questioned him about the discrepancy, Mr. Cortez changed course, saying he had made some of the calls from a Starbucks.
Examining cell phone data is a technique that has moved from being a masterful surprise in trials to being a standard tool in the investigative arsenal of the police and prosecutors, with records routinely provided by cell phone companies in response to subpoenas.
Its use in prosecutions is often challenged, for privacy reasons and for technical reasons, especially when the data comes during the morning or evening rush, when circuits are crowded and calls can be redirected to other towers. But it is often allowed and is used by both prosecutors and defence attorneys to buttress their cases.