Just heard the police are using plate scanners to look for warrants and gather info on protestors please be careful. #Ferguson

— deray mckesson (@deray) October 23, 2014

Often when I talk to reporters about privacy-invasive technologies like license plate trackers, they ask me something along the lines of this question: "Sure, it's possible that the police could abuse this technology, but do you have any evidence that they have done so?" It's a good question, but for a couple of reasons it's usually very difficult for me to answer it.

For one thing, police deploy surveillance technologies mostly in the dark, and usually don't call the press or the ACLU when they discover misconduct in their ranks. At the same time, some of the things the public or the ACLU likely think are inappropriate—for example monitoring First Amendment protected speech—police may think are totally justifiable, for what they call "legitimate law enforcement purposes."

Apart from the question of what kind of surveillance is legitimate, there's a threshold matter: getting any information from police about how they use surveillance technologies like license plate readers can be difficult to say the least. And when advocates and journalists do obtain this information, we usually get only vague, procedural records showing, for example, internal rules and regulations, or the number of license plate records a department holds at a given moment. Hardly ever do we get access to operational information, such as for which kinds of investigations license plate trackers are used. And that's the juicy stuff reporters often ask me for: "Have you any evidence that the cops have used these inappropriately?" For the reasons laid out above, it's an exceedingly frustrating question to answer.

But every now and then there's a crack in the secrecy chamber, and we get a trickle of information pointing us in the right direction. An AP story on the NYPD's spying on Muslims provided one such window with license plate readers. The cops were stationing plate trackers outside mosques to create lists of worshippers who parked their cars in the community lot, according to the story. The Boston Globe reported on another:

In 2004, police tracked Canadian reporter Kerry Diotte via automated license scans after he wrote articles critical of the local traffic division. A senior officer admitted to inappropriately searching for the reporter’s vehicle in a license scan database in an attempt to catch Diotte driving drunk.

Now an activist in Ferguson says he's hearing the police are using license plate readers to track and monitor activists involved in the protests seeking justice for police shooting victim Mike Brown. I'm curious to hear more about this allegation, and hope reporters covering Ferguson do some digging. It would be totally unsurprising to find that it's true. Absent commonsense legal protections to ensure police don't abuse their access to powerful surveillance tools, why wouldn't officers use license plate trackers to keep an eye on protesters who are demanding that one of their own face murder charges? After all, not long ago those very same police officers were told to stop wearing "I Am Darren Wilson" bracelets.