The ACLU has just released the results of our affiliates’ public records requests to hundreds of police departments around the country asking them about their cell phone tracking policies.

What we have learned is disturbing. Many of the approximately 200 law enforcement agencies that responded said they track cell phones without a warrant. As The New York Times reports, this invasive form of surveillance often happens without any court oversight at all.

A small number of agencies, such as in North Las Vegas and Wichita, said they do obtain warrants based on probable cause before tracking. Others, such as the Kentucky State Police, said they use varying legal standards, such as a warrant or a less-strict subpoena. The result is unclear or inconsistent legal standards from town to town that frequently fall short of probable cause.

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.

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.

Last August, in an unprecedented effort to penetrate the secrecy around the policies, 35 ACLU affiliates around the country filed over 380 requests under states' freedom of information laws. The ACLU asked state and local law enforcement agencies about their policies, procedures and practices for tracking cell phones. An in-depth summary of what we found, with links to documents, is here.

The responses varied widely, and many agencies did not respond at all. The documents included statements of policy, memos, police requests to cell phone companies (sometimes in the form of a subpoena or warrant), and invoices and manuals from cell phone companies explaining their procedures and prices for turning over location data. There’s a map with links to the documents and requests state-by-state here.

The documents provide an eye-opening view of police surveillance of Americans. In Wilson County, N.C., police obtain cell phone tracking data where it is “relevant and material” to an ongoing investigation – a standard much lower than probable cause. Police in Lincoln, Neb., without demonstrating probable cause, obtain even GPS location data, which is more precise than cell tower location information. In Tucson, Ariz., police sometimes obtain cell phones numbers for all of the phones at a particular location at a certain time (this practice is known as a “tower dump”).

The U.S. Supreme Court in January held in U.S. v. Jones that prolonged location tracking is a search under the Fourth Amendment, but the effects of that ruling on law enforcement have yet to be seen.

The ACLU supports bipartisan legislation currently pending in both the House of Representatives and the Senate that would address this problem called the Geolocation Privacy and Surveillance (GPS) Act. It would require law enforcement officers to obtain a warrant to access location information from cell phones or GPS devices. It would also mandate that private telecommunications companies obtain their customers’ consent before collecting location data. At least 11 state legislatures are also considering bills related to location tracking.

Technology is evolving quickly, and often to the detriment of privacy. How much privacy Americans enjoy is a choice that ultimately is ours as a society to make.