CHICAGO — A federal lawsuit alleges that a longtime Chicago police officer beat a mentally disabled man while off duty last year, sticking his gun in his mouth and filing a false police report to cover up the incident.

The suit was filed Friday on behalf of Oeinna Jackson, the aunt and legal guardian of 19-year-old Nathaniel Taylor, who has a cognitive disability.

Taylor was treated at a local hospital and sent to the Cook County Jail on charges of assaulting a police officer and resisting arrest, the lawsuit says. He spent a week in jail before being placed on electronic monitoring, which caused severe emotional trauma, according to the suit.

The suit alleges Officer Matthew Jackson, 47, became enraged when Taylor crossed onto the officer’s yard in September 2015.

Richard Dvorak, an attorney representing Oeinna Jackson, showed the Chicago Tribune (http://trib.in/2cEcESb ) photos of the scene that appear to show blood splattered on a walkway outside of Matthew Jackson’s home. Dvorak also provided photos of Taylor taken at the hospital with a bloodied nose, a fat lip and blood stains on his pants.

“He’s not someone who should be subjected to this kind of treatment,” Dvorak said. “All (Taylor) did was cross this officer’s lawn. The officer placed felony charges on this young man to cover up his own misconduct.”

Records show Jackson filed a police report saying Taylor attempted to enter his house and flee before a fight ensued. Jackson said Taylor tried to grab Jackson’s gun from the holster.

Jackson declined to comment on the allegations.

The Cook County state’s attorney’s office and the city’s Independent Police Review Authority, which investigates police misconduct, are looking into the encounter.