Facebook violated the law when it studied the emotions of its users without their knowledge or consent, according to a formal complaint the Electronic Privacy Information Center lodged with the Federal Trade Commission.

The center, based in Washington, D.C., filed the complaint late last week. It alleges that the social media network deceived users when it secretly conducted a psychological experiment designed to determine "if seeing positive or negative updates in their news feeds" would influence how users felt, USA Today reported.

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 states. "The company purposefully messed with people's minds."

The 13-page complaint states that over one week in 2012, approximately 700,000 users' news feeds were intentionally manipulated so that some users saw more news that was happy and others saw more that was sad.

At the conclusion of the study, researches reported that "emotional states can be transferred to others via emotional contagion, leading people to experience the same emotions without their awareness," the study states.

Facebook responded to the complaint with a statement that reads in part:

"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."

Center for Digital Democracy Executive Director Jeffrey Chester reportedly told USA Today he also had contacted regulators about Facebook's practices and may file a complaint.

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

Facebook is already under investigation by British regulators.

Sheryl Sandberg, the company's chief operating officer, apologized Wednesday, saying the study, part of "ongoing research," was "poorly communicated."

Regardless, the experiment has angered Facebook users, many of whom who have posted their displeasure on Facebook itself and on other social media.