(CHICAGO) An off-duty Chicago Police officer was grazed by a bullet early Sunday and returned fire in the Bridgeport neighborhood on the South Side.

The officer completed his shift shortly after 5 a.m. and was driving west on West 31st Street near South Loomis Street when a vehicle sped up behind his vehicle, Chicago Police spokesman Anthony Guglielmi said in a statement.

The officer slowed down, in an effort to let the other vehicle travel around him, but the vehicle pulled up alongside the officer’s vehicle and someone inside pointed a gun in his direction and fired shots, Guglielmi said.

The officer suffered a graze wound to the upper arm, he said. He returned fire and the other vehicle sped away.

He was taken in good condition to Northwestern Memorial Hospital, Guglielmi said, adding that the officer is expected to be treated and released.

The motive for the shooting was not clear Sunday morning as detectives investigate.

It was not known whether anyone was struck when the officer discharged his firearm. The Independent Police Review Authority is investigating the incident, spokeswoman Mia Sissac said.