Jessica Guynn

USA TODAY

SAN FRANCISCO — A privacy watchdog group filed a formal complaint with the Federal Trade Commission claiming Facebook broke the law when it conducted a study on the emotions of its users without their knowledge or consent.

The Electronic Privacy Information Center filed the complaint on Thursday.

The complaint alleges Facebook deceived users by secretly conducting a psychological experiment to explore if seeing positive or negative updates in their news feeds would sway their emotions.

"At the time of the experiment, Facebook did not state in the Data Use Policy that user data would be used for research purposes. Facebook also failed to inform users that their personal information would be shared with researchers," the complaint says.

Facebook issued a statement in response to the complaint.

"When someone signs up for Facebook, we've always asked permission to use their information to provide and enhance the services we offer. To suggest we conducted any corporate research without permission is complete fiction," the statement says. "Companies that want to improve their services use the information their customers provide, whether their privacy policy uses the word 'research' or not."

Jeffrey Chester, executive director of the Center for Digital Democracy, said he has also reached out to regulators and is considering filing his own complaint.

"We are speaking to the FTC next week about the potential violation," Chester said.

Facebook is already facing an investigation from British regulators.

Facebook is under a settlement agreement with the FTC over privacy issues, but the settlement was reached in August 2012 after the one-week study was conducted.

Facebook Chief Operating Officer Sheryl Sandberg apologized on Wednesday, saying the study was part of "ongoing research" to test products and was "poorly communicated."

The experiment has angered Facebook users who have protested on Facebook and social media.

The complaint from the Electronic Privacy Information Center echoed those protests.

"The company purposefully messed with people's minds," it says.