The FBI has invested hundreds of thousands of dollars in license-plate reader technology, but it halted a purchase order at least temporarily after lawyers raised privacy concerns about the surveillance three years ago, according to emails and other documents obtained by The Associated Press.

The heavily redacted emails provide a limited view into some of the internal discussions over the last decade about the technology, which relies on a network of cameras to capture and store data from vehicle license plates. They also indicate that the FBI sought to develop a policy governing its use.

Law enforcement officials view the plate scanners as valuable in tracking the location and movement of suspicious vehicles and as a tool for a broad range of criminal investigations, including kidnapping. But as the number of license tags recorded in police files has grown well into the millions, privacy groups such as the American Civil Liberties Union worry about the technology's ability to pinpoint the locations of innocent motorists. The ACLU received the FBI records through a Freedom of Information Act request.

FBI spokesman Chris Allen said Thursday that the FBI still utilizes license-plate readers, but "they may only be deployed in support of an investigation and only if there's a reasonable belief that they will aid that investigation."

The FBI's Video Surveillance Unit conducted testing of an automatic license plate system over a couple of days in December 2007 and found the technology "very impressive," the documents show. The FBI arranged to buy the equipment from a Greensboro, North Carolina, manufacturer called ELSAG and "invested an estimated 400k in labor to design, develop and test" ELSAG products, according to an internal memo justifying the acquisition.

"Procurement of like systems will allow VSU to rapidly increase the number of units in the field," the document said.

A 2010 purchase request shows that the technology had already been bought in "limited quantities and deployed to numerous field offices."

But a separate email from June 2012 reveals internal uncertainty about how the technology should be used, with the author saying that the FBI's Office of General Counsel was "still wrestling with" license-plate reader privacy issues and that an assistant director had stopped the purchase based on advice from the lawyers. The identities of the email sender and recipients were redacted, and it is not clear from that email alone exactly what the concerns were.

"Once these issues have been resolved ... hopefully this summer ... we expect to be back. The program is still growing and we enjoy tremendous field support," the emails states.

Another email exchange from the following month showed that FBI officials, mindful of the privacy concerns, were working to develop a policy for the technology. But whatever policy was drawn up as part of those discussions was withheld from the documents provided to the ACLU, and it was unclear how the policy is being applied.

Allen, of the FBI, said the system was operated "now with guidance provided that addresses privacy concerns."

The ACLU said the emails still leave unanswered questions, including the extent to which the FBI continues utilizing the technology and for what purpose, and acknowledged that use of the license plate readers has likely changed since the time period covered by the emails.

"While internal discussion is unquestionably a good thing, it is by no means sufficient," Bennett Stein, an ACLU privacy lawyer, wrote for a blog on Friday. "The public has a right to know what information about non-suspects is collected, how long it is retained, whether it is shared with other agencies or departments and for what reasons, and what oversight mechanisms are in place."