He managed to escape the country on a Eurostar train to Paris, although grainy photos of him plastered the walls of the train station.
Yet the police swiftly tracked his escape, for the most mundane of reasons--he did not turn off his cell phone.
Cell phones have, in the past decade, gone from a novel toy to a device nearly as common as a watch. That is a boon to investigators, because a cell phone is also the perfect tracking device, capable of pinpointing any user's location to within as close as a few feet when it is turned on, even if the user is not making calls.
"If your phone is on, they know exactly where you are," said Paul Sagawa, an analyst with Sanford Bernstein, an investment research company, in New York City.
In this case, the police in Italy said Monday that they were told by their British counterparts that one suspect in the July 21 bombing attempts had fled Britain and had made calls to Italian phone numbers in the past. Carlo de Stefano, chief of Rome's antiterrorism police, said the suspect, identified as Hussain Osman, had called family members during his journey from London to Rome.
The calls would have allowed the police to determine where he was when he made them as well as track down where the family members lived by looking up the numbers he dialed. The Italian police determined that Mr. Osman had bought a new prepaid cell phone calling card, which stores a name and number, on July 25. They arrested him on Friday.
Cell phones send out a constant signal, which transmits voice or other data. Some rely on a global positioning system, or G.P.S., which sends the signal to satellites that can pinpoint almost exactly where a user is. Earlier generation phonesto nearby antennas. Using three or more of these antenna transmissions, in a process called triangulation, investigators can determine the user's general location.
"G.P.S. get you within 6 feet and triangulation can get you within 40 feet" of a cell phone user, Mr. Sagawa estimated.
Britain, Europe and the United States have passed laws to take full advantage of those tracking abilities. Law enforcement officials work with cell phone companies to review information about cell phone users they suspect of illegal activity.
In the United States a subpoena is required for that information, in much of Europe a court order, and in Britain the police need to determine that the matter is an issue of national security or required to prevent a crime. Mobile phone companies routinely honor those requests. Verizon has a 24-hour hot line for the police to get user information, for example, and fields thousands of requests a year.
Armed with the proper clearance, law enforcement officials can track whom a suspect is calling, where a suspect moves during a call and where the person ends up--and in some cases, even get records about the people to whom the suspect talked months before. Since the July 7 bombings in London, Britain's home secretary, Charles Clarke, has urged European phone companies to keep cell phone call records for a year.