WASHINGTON -- Federal, state and local law enforcement agencies conducting criminal investigations collected data on cell phone activity thousands of times last year, with each request to a phone company yielding hundreds or thousands of phone numbers of innocent Americans along with those of potential suspects.
Law enforcement made more than 9,000 requests last year for what are called "tower dumps," information on all the calls that bounced off a cell phone tower within a certain period of time, usually two or more hours, a congressional inquiry has revealed.
The little-known practice has raised concerns among federal judges, lawmakers and privacy advocates who question the harvesting of massive amounts of data on people suspected of no crime in order to try to locate a criminal. Data linked to specific cell towers can be used to track people's movements.
The inquiry, by Sen. Edward Markey, D-Mass., into law enforcement's use of cell phone data comes amid growing scrutiny of the bulk collection of geolocation data overseas and of Americans' phone records in the United States by the National Security Agency.
Tower dumps raise in the law enforcement context some of the same concerns presented by the NSA's mass collection of phone records without a warrant, when large amounts of data on law-abiding citizens are gathered to find clues about a small number of suspects, privacy advocates and some industry lawyers say. But unlike the NSA collection, which is bound by court-imposed rules on retention and use, the standards for obtaining tower data and the limits on its use by a plethora of agencies are inconsistent and unclear.
"This isn't the NSA asking for information," said Mr. Markey, who is planning to introduce legislation this month to restrict law enforcement's use of consumers' phone data, including ensuring that tower dumps are narrowly focused. "It's your neighborhood police department requesting your mobile phone data. So there are serious questions about how law enforcement handles the information of innocent people swept up in these digital dragnets."
Mr. Markey's investigation, based on a survey of eight U.S. phone companies, has also revealed that carriers, following requests from law enforcement agencies, are providing a range of other records as well. Those include GPS location data, website addresses and, in some cases, the search terms Americans have entered into their cell phones.
Mr. Markey's legislation, which does not focus on NSA or other intelligence agencies' programs, would require a warrant to obtain GPS location data, impose limits on how long carriers can keep customers' phone data, and mandate routine disclosures by law enforcement agencies on the nature and volume of requests they make of carriers. The proposal would not require a warrant for tower dumps.
Mr. Markey is pushing for standardized rules on law enforcement's use of cell phone data, which has come under question in the courts. One federal judge this year, in a rare public ruling, required a warrant for tower dumps. The judge also ordered the prosecutor to destroy nonrelevant data and -- in an unprecedented move -- to notify all nonsuspects that their records had been collected.
Some companies Mr. Markey surveyed did not provide full responses. But in general, the survey reflects a greater reliance on cell phone data of all types, including text messages and call histories, by law enforcement.
"The industry as a whole in recent years experienced a substantial increase in these demands: the number of requests to Verizon Wireless has approximately doubled in the last five years, a trend that appears to be consistent with the industry in general," the company's general counsel, William Petersen, said in a letter to Mr. Markey.
Civil liberties advocates are alarmed.
"Tower dumps violate the privacy of hundreds or thousands of innocent people at a time, most of whom will never learn their location information was obtained by the government in order to find the proverbial needle in the haystack," said Christopher Soghoian, principal technologist at the American Civil Liberties Union.
According to the industry, there are 330 million cell phones in use in the United States. Americans are texting, calling and moving about with devices that track their locations and enable them to surf the Internet. Those actions leave digital trails that companies log for business or technical reasons -- and that can be invaluable for investigators on the trail of a murderer or a bank robber.
"Location information is a vital component of law enforcement investigations at the federal, state and local levels," FBI spokesman Christopher Allen said.
A senior Justice Department official, who was not authorized to talk on the record and so spoke on the condition of anonymity, said: "The data is useful if you think about the need to identify the location of drug traffickers -- where are they at various points in their criminal dealings? It would be useful if you have a shooting by gang members. How do you figure who was at the scene of the shooting?"
In one notable instance, the FBI in 2010 located two bank robbers in Arizona, whom prosecutors had dubbed the "High Country Bandits," through the use of tower dumps. The men had robbed 16 banks at gunpoint in Arizona, New Mexico, Colorado and Utah.
Investigators traced the trail of the robbers by analyzing cell phone records from the towers closest to four of the more remote Arizona banks.