U.S. Senator Al Franken has in a letter asked the Department of Justice for information on its practices in requesting location information from wireless carriers, following reports that law enforcement agencies are requesting such information sometimes without warrants.

Franken, a Minnesota Democrat, said he was concerned about reports that after a Supreme Court decision on tracking using GPS devices, state and local law enforcement agencies may be requesting the location records of individuals directly from their wireless carriers instead of tracking individuals through GPS devices installed on vehicles, according to a copy of the letter on the website of the American Civil Liberties Union.

The Supreme Court ruled in January that the use of GPS devices to track individuals amounted to a search and required a warrant under the Fourth Amendment of the U.S. Constitution, which protects individuals from unreasonable searches.

U.S. law enforcement agents collected information by installing a GPS device on the vehicle of Antoine Jones, a Washington nightclub owner, that lead to his conviction on drug trafficking conspiracy charges.

"I am eager to learn about how frequently the Department requests location information and what legal standard the Department believes it must meet to obtain it," Franken said in the letter addressed to attorney general Eric Holder which is dated May 10. Franken also wants to know how the Department may have changed its practices after the Supreme Court decision.

Congressman Edward J. Markey, a Democrat from Massachusetts, sent last Thursday a letter to nine major wireless carriers, including AT&T, Verizon and T-Mobile, asking them about their policies and practices for sharing their customers' mobile phone information with law enforcement agencies, including whether they consider if court warrants have been obtained. Markey also asked if the carriers accepted payments and other forms of compensation in exchange for the information provided.

The tracking of cellphones by law enforcement, once used mainly by federal agents, is widely used as a surveillance tool by local police officials, who often use it with "little or no court oversight," The New York Times reported in March. The carriers charged for the information, the newspaper added.

Many of the about 200 law enforcement agencies, responding to public record requests by the ACLU, track cellphones without a warrant, it said in April. The information was gathered under freedom of information laws, it said.

John Ribeiro covers outsourcing and general technology breaking news from India for The IDG News Service. Follow John on Twitter at @Johnribeiro. John's e-mail address is john_ribeiro@idg.com