The Senate Judiciary Committee wants more answers about law enforcement agencies across the country deploying surveillance technology, including trick cellphone towers, that gather cellphone data, according to a letter obtained Thursday by

The bipartisan letter was sent to the departments of Justice and Homeland Security, following a recent FBI policy change regarding search warrants that committee leaders say raises questions about privacy protections and how the equipment was used.

Among the tools singled out in the letter is a Stingray, a device that pretends it is a cellphone tower and tricks cellphones into identifying some of their owners' account information.

In addition, the U.S. Marshal Service is deploying an airborne device -- called a “DRT box” or “dirtbox” -- from five metropolitan-area airports across the United States that also “mimic standard cell towers, forcing affected cell phones to reveal their approximate location and registration information,” the Dec. 23 letter states.

“It remains unclear how other agencies within the Department of Justice and Department of Homeland Security make use of cell-site simulators and what policies are in place to govern their use of that technology,” states the letter from Vermont Democratic Sen. Patrick Leahy, the committee chairman, and Iowa Sen. Chuck Grassley, the top Republican on the committee, reported first by The Associated Press.

The letter also states: "The Judiciary Committee needs a broader understanding of the full range of law enforcement agencies that use this technology, the policies in place to protect the privacy interests of those whose information might be collected using these devices, and the legal process that DOJ and DHS entities seek prior to using them."

A Justice Department spokeswoman told the wire service that agency officials are reviewing the letter.

Law enforcement authorities have said the technology, which allows police to obtain cellphone information without having to ask for help from service providers, is useful for catching criminals, though civil liberties advocates have raised privacy concerns.

The senators’ letter says that FBI officials in recent months have told committee staffers that the agency recently changed its policy so that it now generally seeks a search warrant before using the cell-site technology but with certain broad exceptions -- such as cases that involve a fugitive, pose an imminent public safety danger or in which the technology is used in a public place where no expectation of privacy would exist.

The senators demanded answers about how the FBI and other law enforcement agencies protect the privacy of people whose cellphone information is collected, even when they're not targeted or suspected of any wrongdoing. The letter had a list of questions, including ones about how often the technology has been used and about how often law enforcement has requested a search warrant.

The FBI confirmed that officials had met with committee staff members and said it would respond to oversight questions but otherwise referred questions about the letter to the Justice Department.

The Associated Press contributed to this report.