A number of organizations, including ProPublica, have developed tools to let the public see exactly how Facebook users are being targeted by advertisers.

Now, Facebook has quietly made changes to its site that stop those efforts.

ProPublica, Mozilla and Who Targets Me have all noticed their tools stopped working this month after Facebook inserted code in its website that blocks them.

“This is very concerning,” said Sen. Mark Warner, D-Va., who has co-sponsored the Honest Ads Act, which would require transparency on Facebook ads. “Investigative groups like ProPublica need access to this information in order to track and report on the opaque and frequently deceptive world of online advertising.”