The accidental release triggered immediate doubts about whether the police could reliably protect the sensitive data. It also raised questions about whether police were following up on the scans, since numerous vehicles repeatedly triggered alarms for the same offenses. One motorcycle that had been reported stolen triggered scanner alerts 59 times over six months, while another plate with lapsed insurance was scanned a total of 97 times in the same span.

The police inadvertently released to the Globe the license plate numbers of more than 68,000 vehicles that had tripped alarms on automated license plate readers over a six-month period. Many of the vehicles were scanned dozens of times in that period alone.

The Boston Police Department has indefinitely suspended its use of high-tech scanners that automatically check whether drivers have outstanding parking tickets, lapsed insurance or other violations after a Globe investigation raised serious privacy concerns.


"We just took [the scanner program] off-line while the commissioner reviews it," said Boston police spokeswoman Cheryl Fiandaca. Commissioner William Evans "wants to review it so he knows that it's being used effectively and that it doesn't invade anyone's privacy."

But privacy advocates said Boston's problems with the scanners underscore how easily the technology can be misused. The Boston police are one of the few departments in the state with explicit policies to protect privacy, but the released data calls into question how closely they follow their own rules.

"It's not realistic to think that law enforcement will police itself when it comes to technologies like license plate readers," said state Representative Jonathan Hecht, a Watertown Democrat who has filed a bill to regulate use of scanners and the sensitive data they collect.

Hecht believes that the scanner technology has "gotten ahead of thoughtful policymaking on its use. . . . From their point of view, more information is always better."


Officials at the American Civil Liberties Union of Massachusetts, which has raised concerns about the proliferation of plate scanners, praised Boston police for suspending the program and urged other departments to follow suit. "We call on police departments statewide to cease using license plate recognition technology until the state Legislature passes regulation," said Kade Crockford of the civil liberties union.

More than 60 law enforcement agencies across Massachusetts use automated license plate recognition technology, including every police department in the Boston area. The scanners use high-speed cameras to compare plates against police databases, including vehicles associated with outstanding warrants, lapsed registration, expired insurance, or unpaid parking tickets.

The readers also record the date, time, and GPS location of each vehicle, even in heavy traffic. The technology thus offers a wealth of information for surveillance as well as investigations: with enough scans over time, police can trace a particular vehicle's path and discern driving habits.

Boston police started with just one experimental scanner in 2006, but expanded to a total of 14 this spring, giving the department the capacity to scan as many as 4 million vehicles a year. However, department officials confirm that the program has never been audited to determine how well it works or follows the privacy policy.

Investigators at MuckRock, a public-records group that works with the Globe, initially requested the police scan data last January. After initially denying the request, Boston police agreed in April to release a database of plates that had triggered alarms, but without individual plate numbers.


But the records finally released in July were unredacted, revealing full plate numbers and GPS location data for more than 40,000 different vehicles, most of which belonged to private citizens.

MuckRock and the Globe brought the inadvertent disclosure to police attention beginning in September. But it was not until late November that department officials acknowledged the error and asked for the information's return. The Globe declined, but has no intention of publishing any individual plate information.

Hecht considers disclosure of private license plate numbers "a fundamental problem."

"The fact that all of this data was released, including personal data," said Hecht, "at a minimum says that the police don't have an adequate oversight system in place."

Beyond providing snapshots of where thousands of vehicles were spotted at given moments, the data indicate that Boston police routinely failed to follow up on repeat alarms.

Nearly 1,700 plates registered five or more scanner hits over the six-month period, most for insurance violations or unpaid traffic fines. The most-scanned plate came back as a hit for lapsed insurance more than 90 times.

But some repeat alarms were for serious violations. 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.

It is unclear what Boston police have done with their mountain of scans, in part because police did not keep records of follow-ups on the data. Fiandaca said that making sure the information is used effectively will be part of Evans's review.

But the ACLU's Crockford said the unanswered questions make her suspicious about the program's purpose. "You can't help but wonder whether the real purpose is simply to collect droves of data about where innocent people are driving, in case it might be useful for investigations later," she said.

The Globe and MuckRock previously published a statewide investigation into license readers, finding that fewer than a third of departments using the scanners had any policy to govern use of this potentially revealing data. Boston does have a formal policy.

Boston's release of six months of data appeared to violate the department's rule that it would only keep records for three months. But Fiandaca said that was only because the Globe's data request took so long to resolve that they collected plate records on two occasions, preserving three months of records each time.


Hecht's License Plate Privacy Act would slash the plate scan retention period to 48 hours except by court order and require agencies to report annually on their scanner use.

"If you go too far in collecting information just because you can, it undermines people's confidence in government," said Hecht. "That ultimately makes law enforcement's job much more difficult."