A 24-year-old Arlington woman was sexually assaulted by her roommate Tuesday night, police said.

At about 11:50 p.m., 31-year-old Allister Kennedy entered his roommate’s bedroom while she slept, pinned her down and sexually assaulted her, according to police.

“She woke up and he was over top of her and he pinned her down,” Arlington police spokesman Dustin Sternbeck said. “She fought him off and contacted 911.”

Both roommates were taken to the police station and interviewed, Sternbeck said, which provided enough information to arrest Kennedy and charge him with animate object penetration. He is being held without bond.

Police could not say how long the two had lived in the apartment in the 4500 block of S. Four Mile Run Drive. No one else lives in the apartment. Neither drugs nor alcohol were involved in the assault, Sternbeck aid.