SAN FRANCISCO (Reuters) - Facebook Inc FB.O barred software developers on Monday from using the massive social network's data to create surveillance tools, closing off a process that had been exploited by U.S. police departments to track protesters

3D-printed models of people are seen in front of a Facebook logo in this photo illustration taken June 9, 2016. REUTERS/Dado Ruvic/Illustration - RTSGTLZ

Facebook, its Instagram unit and rival Twitter Inc TWTR.N came under fire last year from privacy advocates after the American Civil Liberties Union (ACLU) said in a report that police were using location data and other user information to spy on protesters in places such as Ferguson, Missouri.

In response to the ACLU report, the companies shut off the data access of Geofeedia, a Chicago-based data vendor that said it works with organizations to “leverage social media,” but Facebook policy had not explicitly barred such use of data in the future.

“Our goal is to make our policy explicit,” Rob Sherman, Facebook’s deputy chief privacy officer, said in a post on the social network on Monday. He was not immediately available for an interview.

The change would help build “a community where people can feel safe making their voices heard,” Sherman said.

Racially charged protests broke out in the St. Louis suburb of Ferguson in the aftermath of the August 2014 shooting of black teenager Michael Brown by a white police officer.

In a 2015 email message, a Geofeedia employee touted its “great success” covering the protests, according to the ACLU report based on government records.

Representatives of Geofeedia could not immediately be reached for comment on Monday. The company has worked with more than 500 law enforcement agencies, the ACLU said.

Geofeedia Chief Executive Officer Phil Harris said in October that the company was committed to privacy and would work to build on civil rights protections.

Major social media platforms including Twitter and Alphabet Inc's YouTube GOOGL.O have taken action or implemented policies similar to Facebook's, said Nicole Ozer, technology and civil liberties policy director at the ACLU of Northern California.

Ozer praised the companies’ action but said they should have stopped such use of data earlier. “It shouldn’t take a public records request from the ACLU for these companies to know what their developers are doing,” she said.

It was also unclear how the companies would enforce their policies, said Malkia Cyril, executive director of the Center for Media Justice, a nonprofit that opposes government use of social media for surveillance.

Inside corporations, “is the will there, without constant activist pressure, to enforce these rules?” Cyril said.