By Chris Arsenault / Al Jazeera

For years, legal scholar Susan Freidwald has been raising alarms about police tracking the locations of average American cellphone users, without warrants or judicial oversight.

But the extent of routine surveillance on the whereabouts of millions of people, outlined in a new report from the American Civil Liberties Union (ACLU) based on 5,500 pages of internal records from 205 police departments across the US, shocked even seasoned observers like Freidwald.

“I am surprised agents are getting information from all the calls to a particular cellphone tower,” Friedwald, a law professor at the University of San Francisco, told Al Jazeera. “I think that is one of the biggest intrusions.”

Cellphones register their location with phone networks several times each minute. This function cannot be turned off when the phone is getting a wireless signal. Police ask phone companies to provide them with data on the phone communicating with the tower, allowing police to monitor the movements of cellphone users, which includes virtually everyone.

In Tucson Arizona, for example, police sometimes obtain cellphone numbers for all the phones in a particular area, allowing innocent people to have their personal information and movements potentially scrutinised by authorities.

‘Casting a huge net’

“In order to track one person who may have committed a crime, law enforcement [in several towns where all calls to a particular tower have been traced] got the whereabouts of hundreds of innocent people,” Catherine Crump, an ACLU attorney, told Al Jazeera.

There are also ways to track a cellphone which is turned off, she said, but that requires more extensive steps from law enforcement agents.

“They are casting a huge net, getting very detailed information that scoops up a lot of innocent people. It has been happening since the late 1990s and is not a particularly democratic way of doing things,” she said.

Police departments, unsurprisingly, disagree with this assessment.

“It’s pretty valuable, simply because there are so many people who have cellphones,” Roxann Ryan, a criminal analyst with Iowa’s state intelligence branch, told the UPI news service. “We find people and it saves lives.”

Jack Lewis, chief of police in Apex, North Carolina, told Fox News:  “We had only used cellphone data to try and locate reported missing persons during active cases where there was (sic) concerns for the missing persons’ welfare.”

Some companies, including AT&T, Verizon and others, have created manuals for police explaining how officers can access the data they store. Exact cost figures were not available in the ACLU report, but location tracking for a phone number is thought to cost departments a few hundred dollars, depending on the specifics of the case.

Tired of relaying on companies, police in the small town of Gilbert, Arizona, spent $244,000 on their own cellphone tracking equipment, so they wouldn’t have to pay cellphone companies to provide data on the movements of their clients.

“Cellphone tracking has become a routine law enforcement technique and has been happening without a warrant,” Crump said.

The fourth amendment of the US Constitution prohibits unreasonable searches and seizures and critics say local police  routinely violate this rule when they track people’s movements.

In January 2012, the US Supreme Court ruled that police overstepped the law by placing a global positioning system (GPS) tracker on the car of an alleged drug dealer without a warrant.

Privacy advocates say the decision US vs Jones means that cellphone tracking, arguably more intrusive than placing a device on someone’s car, should certainly require a warrant.

“We have a finely honed set of rules about what the police can do in the physical world,” Friedwald said. “Those principals haven’t really translated into the world of mobile technology.

Read more from Al Jazeera: http://www.aljazeera.com/indepth/features/2012/04/20124518519786175.html