When Facebook Graph Search launched six years ago, it was meant to help users discover content across public posts on the platform. Since then, the feature stayed relatively low-profile for many users (its last major announcement was in 2014 when a mobile version was rolled out), but became a valuable tool for many online investigators who used it to collect evidence of human rights abuses, war crimes and human trafficking. Last week, however, many of them discovered that Graph Search features had suddenly been turned off, reports Vice.

Graph Search let users search in plain language (i.e. sentences written the way people talk, not just keywords), but more importantly, it also let them filter search results by very specific criteria. For example, users could find who had liked a page or photo, when someone had visited a city or if they had been in the same place at the same time with another person. Despite the obvious potential for privacy issues, Graph Search was also an important resource for organizations like Bellingcat, an investigative journalism website that used it to document Saudi-led airstrikes in Yemen for its Yemen Project.

Other investigators also used Graph Search to build tools like StalkScan, but the removal of Graph Search means they have had to suspend their services or offer them in a very limited capacity. For example, StalkScan’s website now has a notice that says:

“As of June 6th, you can scan only your own profile with this tool. After two years and 28M+ StalkScan sessions, Facebook decided to make the Graph Search less transparent. As usual, they did this without any communication or dialogue with activists and journalists that used it for legitimate purposes.The creepy graph search itself still exists, but is now less accessible and more difficult to use. Make sure to check yourself with this tool, since your data is still out there!”

Facebook may be trying to take a more cautious stance because it is still dealing with the fall out from several major security lapses, including the Cambridge Analytica data scandal, as well as the revelation earlier this year that it had stored hundreds of millions of passwords in plain text.

In a statement to Vice, a Facebook spokesperson said “The vast majority of people on Facebook search using keywords, a factor which led us to pause some aspects of graph search and focus more on improving keyword search. We are working closely with researchers to make sure they have the tools they need to use our platform.” But one of Vice’s sources, a current employee at Facebook, said within the company there is “lots of internal and external struggle between giving access to info so people can find friends or research things (like Bellingcat), and protecting it.”

TechCrunch has contacted Facebook for more information.