Many US police departments use cell phone tracking, often without court orders, to find suspects and investigate criminal cases, according to a study released Monday.

The survey released by the American Civil Liberties Union found that "the overwhelming majority of law enforcement agencies that responded engage in at least some cell phone tracking," the organization said.

"Most law enforcement agencies that responded engage in cell phone tracking for investigative purposes. Even those that have not tracked cell phones in the course of a criminal investigation have tracked cell phones in emergencies, for example to locate a missing person."

The use of phone tracking, using GPS or other technology to locate people through their cell phones, is a murky legal area.

The US Supreme Court has held that the use of GPS devices placed by police on a suspect's car constitutes an "unreasonable search" under the constitution. But the question of cell phone tracking is still making its way through the courts.

Several members of Congress have introduced bills calling for "location privacy" to be respected by police, except in cases of emergency.

The ACLU said its survey of more than 200 law enforcement agencies showed "disturbing" results, with few police departments seeking warrants and "unclear or inconsistent legal standards" depending on the jurisdiction.

"What we have learned is disturbing. The government should have to get a warrant before tracking cell phones. That is what is necessary to protect Americans' privacy, and it is also what is required under the constitution," said Catherine Crump, an ACLU attorney.

"The fact that some law enforcement agencies do get warrants shows that a probable cause requirement is a completely reasonable and workable policy, allowing police to protect both public safety and privacy."

The ACLU said it began seeking data last year, filing over 380 requests under states' freedom of information laws.

The responses varied widely, and some agencies did not respond at all.

The civil liberties group said the tracking can be lucrative for mobile operators, and that many charge fees to law enforcement to provide tracking data.