FAIRFAX, Va. (AP) — A federal lawsuit has been filed against a Fairfax County police officer over the use of a stun gun on a man who says he has cerebral palsy.

The lawsuit filed Thursday says the use of a stun gun on Elton Cansler in September 2015 was “grossly excessive” and “objectively unreasonable.”

Video taken by a bystander shows the officer, who is white, shocking Cansler in the back while his hands are on a police car.

Police have defended the use of force, saying Cansler was armed with a knife and kept reaching for it.

Cansler, who is black, was charged with assaulting a police officer and larceny. The assault charge was later dropped.

A police spokesman referred questions to the county attorney’s office, which didn’t immediately return a message.