Police said in a statement released Friday night that the alleged assault occurred in a city hotel room early Sunday after the 49ers' playoff victory over the Green Bay Packers.
The statement says Crabtree has been interviewed with his attorney present and has cooperated with the probe. The receiver hasn't been detained or arrested, and agreed to be available for further questions in the future.
Authorities didn't release any further details. A spokesman for the 49ers was traveling with the team to Atlanta and couldn't immediately comment.
San Francisco is preparing to play the Atlanta Falcons in the NFC championship game on Sunday. The winner goes to the Super Bowl.