The Boston Police Department (BPD) has indefinitely halted its use of license plate readers (LPR) following an investigation into their use published on Saturday by the investigative journalism organization MuckRock and the Boston Globe.

David Estrada, a BPD spokesperson, confirmed to Ars that the department had stopped using its LPRs for now and asked that further questions be submitted in writing, which Ars has done.

This suspension likely makes Boston one of the largest cities in America to stop using this sort of technology, which for years has been in wide use by thousands of law enforcement agencies nationwide. At present, BPD uses 14 individual LPRs, which enables them to scan “as many as 4 million vehicles a year,” the Globe noted.

As Ars has reported for more than a year, LPRs are in use in cities big and small across America. Typically, the specialized cameras scan a given plate using optical character recognition technology, checking that plate against a “hot list” of stolen or wanted vehicles. The device then also typically will record the date, time, and GPS location of any plates—hot or not—that it sees.

The cameras typically scan at an extremely high rate, usually around 60 plates per second. Law enforcement policies vary widely as to how long that information can be retained. Different agencies keep that data anywhere from a few weeks to indefinitely. Some cities have even mounted such cameras at their city borders, monitoring who comes in and out.

In 2012, reporting by MuckRock showed that BPD could keep such data “indefinitely for investigatory purposes,” although data is otherwise routinely kept for 90 days. In April 2013, MuckRock further reported that just 32 percent of Massachusetts law enforcement agencies known to operate LPR deployments have formal standards governing the use of the scanners and duration of their data retention.

BPD scanning its own vehicles

The whole saga kicked off when MuckRock requested an LPR dataset from the BPD in January 2013. The request was initially denied, but then by April 2013, the BPD agreed to release a database of scans that triggered flags, without the actual plate numbers. But when the data was actually handed over, the BPD inadvertently included unredacted data on 68,000 vehicles across six months.

The two news agencies brought this disclosure to the attention of the BPD in September 2013, but it was not until November 2013 that they acknowledged the error and asked for the data to be returned. The Globe declined to do so, “but has no intention of publishing any individual plate information.”

The Globe’s own cursory analysis shows that some vehicles were scanned dozens of times despite being flagged for being stolen or having expired insurance. This data set also appears to raise even more questions about the use of the LPR infrastructure.

As the Globe reported:

One Harley Davidson motorcycle that had been reported stolen passed license plate scanners a total of 59 times between Oct. 19, 2012 and March 13, 2013. It was often recorded on sequential days or multiple times in a single day, all by the same scanner and almost always within the same half-hour span in the early evening. Boston police chief technical officer John Daley indicated that each of these scans prompted an e-mail alert to the department’s Stolen Car Unit, but there is no indication that the motorcycle was ever apprehended or even stopped. Some of the most frequent hits in the database were scanned in Boston police’s own employee parking lots. More than two hundred vehicles parked in the police substation lot in South Boston, a mix of official and personal vehicles, triggered scanner alerts over the six months. Police declined to discuss why they would be scanning the parking lot or why there would be so many potential violations.

Collect all the plates

Politicians and advocacy groups, chiefly the American Civil Liberties Union of Massachusetts, say that this new revelation is highly disturbing.

“The main thing here is that police were not following up about stolen cars,” Kade Crockford, the director of the Technology for Liberty Initiative at the ACLU Massachusetts, told Ars.

She noted that typically, police departments tout LPRs as a way to help vastly expand an agency’s ability to catch stolen vehicles. Crockford also underscored that her organization is not opposed to LPRs in principle, but called for tighter and more uniform regulations about how long LPR data is retained and under what circumstances it can be used.

The national office of the ACLU also recently published an analysis showing that of the over 204 million plates that were scanned in Washington, DC, during 2012, just 0.01 percent of them even registered as being on the “hot list,” which can range from a minor infraction to being wanted for a major crime.

“It’s clear that in fact the reason that police departments are using this technology might not actually be the reason that they say they’re using it for,” Crockford added. “We found that in this case they’re not following up on the stolen car hits and to us that says: the only [reason] that it’s being used is to collect huge troves of people’s movements.”