The FBI has deployed surveillance technology that can read car license plates around the country despite its own internal worries about the privacy implications of the mass tracking devices, newly released documents reveal.

The paper trail, obtained by the American Civil Liberties Union under freedom of information laws, shows how the FBI has used the technology – known as automatic license plate readers or LPRs – in several of its field offices. But the documents also reveal that privacy concerns have been raised within the agency itself that temporarily put a halt to the practice.

An email exchange between FBI agents dated June 2012 records that the assistant director of the FBI postponed the purchase of a particular type of camera linked to LPRs after he was advised by his own legal department, the office of general counsel (OGC), of privacy concerns. The document is redacted, thus obscuring the precise nature of the camera, but it does note that the OGC is “still wrestling with LPR privacy issues”.

Another batch of documents obtained by the ACLU records the interest of the FBI’s Video Surveillance Unit (VSU) in LPRs stretching back some time. One document notes that the “VSU has spent years evaluating LPR products”.

Later in the same document it is revealed that LPRs were bought in “limited quantities and deployed to numerous field offices”. The equipment was purchased from a company called ELSAG North America headquartered in Brewster, New York, that claims to be able to record up to 1,800 license plates a minute with cameras mounted on police cars or on fixed points such as bridges or overpasses.

The documents show that the FBI has worked directly with the company to help it develop its capabilities, investing about $400,000 in the exercise.

A spokesperson for the FBI confirmed to the Guardian that the agency was deploying license plate readers around the country, but stressed that they were only used in already active investigations and “only when there’s a reasonable belief that the LPR will aid that investigation”.

He said there was no general data grab of random license plate details or storage of such information in bulk databases. With the help of the OGC, the agency had produced guidance governing the use of LPRs “that addresses privacy concerns”.

The ACLU and other civil liberties groups have long been concerned about the proliferation of LPRs as a form of law enforcement surveillance. Privacy activists have focused on the practice of many police departments of indiscriminately scraping data from thousands of cars and storing it indefinitely.

Multiplied across the US, the ACLU has calculated that millions of Americans are subject to the surveillance, from which at its most extreme profiles of an individual’s movement, behavior and associations could be created.

Jay Stanley, the ACLU’s expert on technology-related privacy issues, said that the heavily redacted documents released by the FBI left many questions still unanswered. “As is so often the case, we are left with the feeling that the public should know more about the policies that the FBI has developed – if the agency has guidance relating to privacy concerns over this very sensitive technology, then the public should be told about it.”

Material in the documents that might indicate what kind of investigations LPRs are deployed in has also been redacted from the documents. The ACLU has raised the possibility that the devices could have been used in controversial surveillance of Muslim communities post-9/11 or to gather intelligence on protesters, though no information exists to confirm the suspicion.