Three St. Louis police officers were indicted on federal charges Thursday for allegedly beating a fellow police officer working undercover and a fourth for allegedly aiding in a cover-up.

In the indictment, federal prosectors say the officers expressed "excitement about using unjustified force against [protesters] and going undetected while doing so," according to the St. Louis Post-Dispatch. The officers allegedly did not know the man was an undercover officer at the time of the incident.

Prosecutors say officers Dustin Boone, Randy Hays and Christopher Meyers kicked and beat a fellow officer with a police baton last September while the officer was working a protest downtown, following the acquittal of former officer Jason Stockley.

Stockley was found not guilty in the fatal shooting of Anthony Smith.

The indictment claims that the three men assaulted their undercover colleague, apparently believing him to be a protester, "while he was compliant and not posing a physical threat to anyone."

A fourth officer, Bailey Colletta, is accused of lying to a federal grand jury in an attempt to cover up the incident.

The three officers who beat the undercover officer, identified only by the initials "L.H.," then allegedly attempted to lie about the incident and claim "L.H." was resisting arrest.

"These are serious charges and the vigorous enforcement of civil rights is essential to maintaining public trust in law enforcement," U.S. Attorney Jeff Jensen said in a statement, according to the Associated Press.

St. Louis Police Officers Association Business Manager Jeff Roorda told the AP all four officers are members of the association and they will be providing them legal assistance.

St. Louis public safety director Jimmie Edwards, who called the officers "outliers" in a statement, said the four officers have been suspended without pay from the police force.