SOMEONE strangled a prostitute in Portland, Oregon in 2002. The police arrested Lisa Roberts, the victim’s ex-lover, who spent more than two years in custody awaiting trial. Shortly before the trial the prosecutor told Ms Roberts, via her lawyer, that tower data collected by Verizon, her mobile-telephone network, showed precisely where she was at the time of the murder. As her lawyer recalled, the prosecutor said Ms Roberts could be “pinpointed” in a park shortly before the victim’s naked and sexually assaulted corpse was found there. She was told she faced 25 years to life in prison. She accepted a deal to plead guilty and serve 15 years.

But the high-tech evidence against her was bunk. Routinely collected tower data can place a mobile phone in a broad area, but it cannot “pinpoint” it. That would require a special three-tower “triangulation”, which cannot reveal past locations. It took a decade for Ms Roberts’s guilty plea to be thrown out. On May 28th she left prison, her criminal record clean, after nearly 12 years in custody.

Mobile-phone-tower location data is widely used in court, says Steven Jansen of the Association of Prosecuting Attorneys in Washington, DC. Last year America’s courts and law-enforcement agencies served 37,839 subpoenas, court orders, and search warrants for location data just to one phone company, AT&T. This year’s figure had already climbed to 30,886 by the end of June, the company says.

In the past couple of years defence lawyers have brought in more experts to challenge tower evidence, says Mr Jansen. Most police officers and lawyers have “no clue” how phone networks function, says Ryan Bialas, a North Dakota public defender. Cop shows like “CSI” give juries the idea that scientific evidence is almost infallible. But tower data is far less accurate than prosecutors sometimes claim.

Location, location, location

Cherry Biometrics, a Virginia forensics firm, has testified in more than 20 trials where tower evidence was being used. In all of them, says Michael Cherry, the boss, prosecutors misunderstood the technology. Often they would assert that mobile phones connect to the nearest tower. In fact, two calls dialled consecutively from the same spot may connect to two different towers: one close by, the other many miles away. Defence lawyers sometimes use the same “junk science” to support alibis, adds Mr Cherry.

The tower cited in Ms Roberts’s case could have handled calls from as far as 20 miles (32km) away, according to a deposition prepared by Cherry Biometrics. Which tower a phone connects with depends on such factors as how thick the nearby foliage and walls are, the size of nearby cars and bodies of water, and how well the handset is working. None of this information is usually recorded.

Without it, much of the tower evidence presented in court is useless, writes Larry Daniel of Guardian Digital Forensics in a forthcoming book on the subject. Mr Daniel has found errors in nearly half of the 240 consulting jobs he has done for prosecutors and defence lawyers. Even so, tower records do have a place in court, says Mr Daniel: they can generally prove that a handset has moved between broad areas such as non-adjacent cities.

Preventing the misuse of tower data should not be hard, though it can be expensive. Guardian Digital Forensics charges $250 an hour for its expertise. The judge who freed Ms Roberts faulted her lawyer, now dead, for failing to challenge mobile-phone evidence that would probably have collapsed before a jury. William Teesdale, the chief investigator for Portland’s federal defender’s office, complains that the prosecutor, Rod Underhill, never addressed another call on Ms Roberts’s phone around the same time as the one that supposedly placed her at the murder scene. It connected to a different tower, farther away from the park and closer to a spot where Ms Roberts had been seen by a witness. Mr Underhill refused to comment.