A masked man hit a University of Michigan student in the back of the head with a baseball bat near campus Monday night, then stole his backpack, Ann Arbor police said.

The attack in the 500 block of Monroe Street left the victim with a bruise, but he refused medical treatment, a police report said.

Police responded after a witness to the robbery called 911 at 11:23 p.m. Detectives are investigating, police spokeswoman Lt. Renee Bush said.

Police say it appears the 20-year-old victim was targeted at random.

He told police he was walking along the south side of the street when he heard footsteps behind him. He was struck in the back of the head with the bat and knocked to the ground, the report said.

The attacker demanded his backpack and continued to hit him, police said. According to the report, he threatened to seriously harm the victim if he didn't hand the bag over.

The victim called out for help, but did not get any response, the report said. He was able to fend off the attacker, who took the bag and left in a red sedan, police said. The sedan was seen traveling east toward South State Street.

According to a University of Michigan police news release, the attacker was seen exiting a passenger-side door of the car prior to the robbery.

The police report does not mention anyone else being seen in the car. The incident occurred off campus across from South Quad.

Police say the backpack contained two calculators and a textbook.

The attacker was described as white, 5 feet, 7 inches tall and of a medium build. He was wearing a black ski mask and black hooded sweatshirt.

Anyone with information can call the department’s confidential tip line at (734) 794-6939.