The Massachusetts State Police Department tweeted a screenshot of a computer monitor Thursday to help illustrate the extent of a series of gas explosions and fires that had occurred throughout Lawrence, Andover, and North Andover, Massachusetts. One person has died and about 20 more are injured, according to WBUR, Boston’s public radio station.

The screenshot of a browser, which has now been deleted, included a map of the affected area, an update on the number of homes impacted, and a warning that anyone who smells gas should evacuate immediately. But that’s not all the tweet shared. The image appears to be a photo taken by a smartphone of a computer screen at the police department, and the browser containing the map had a row of bookmarks—bookmarks that include pages of left-wing activist Facebook groups. Oops?

The Massachusetts State Police deleted a tweet featuring this image, highlighting the houses effected by a gas explosion on the map. Can you tell why it was deleted? The police were spying on Left-Wing groups which you can see on their favorited list. pic.twitter.com/EDUEk6zU0J — Collin Fischer (@CollinFischer86) September 14, 2018

While no activist should be surprised that police are tracking them, the concentration of left-wing activist groups at the top of the browser, ostensibly to navigate to the pages regularly and quickly, was a rare if inadvertent official acknowledgement of this kind of tabs-keeping. The Facebook groups that were bookmarked include Massachusetts Action Against Police Brutality (MAAPB), which organizes to end police brutality and shares stories of police killings, the Coalition to Organize and Mobilize Boston Against Trump (COMBAT), and a link to a website called “Resistance Calendar,” which posts Trump-opposition events across the country. There did not appear to be tabs on this particular computer for right-wing political groups that organize online, but perhaps those bookmarks could be found on another department computer.

In a statement to WBUR, the state police didn’t deny their surveillance of activist groups, but added that they monitor public gatherings. “We, obviously, need to know if large numbers of people, for whatever reason, are going to be on public roadways or public spaces, so that we may ensure the safety and rights of those who have gathered as well as of the members of the public around them,” the state police said. Neither MAAPB nor COMBAT appear to have any upcoming events, though.

It seems like there are lessons here for everyone. For activists, particularly those who criticize police or the political party currently in power, it’s a reminder that indeed they are being watched and might want to do more of their organizing in more encrypted, privacy-protecting place. And for the cops, while it’s important to be transparent, maybe take a beat to crop your screenshots before posting them to social media, especially if they contain people’s personal information, like activist names and photos for example. You don’t even need to snap a smartphone pic. Command-shift-4: keystrokes to live by.