A jogger who was knocked unconscious by an assailant on the Capital Crescent trail in Bethesda Sunday night has spoken to detectives in the hospital, and police are investigating the crime as a sexual assault, authorities said Monday.

The 21-year-old woman told officers she was running on the trail between Massachusetts Avenue and MacArthur Boulevard when she stopped to watch deer cross the path. She was hit on the head from behind about 7 p.m. Police say she regained consciousness in the woods and yelled for help. She is in serious condition with a head injury.

The suspect was described as a man in his 20s who was wearing dark clothes, police said. They asked anyone with information to call 240-773-5070.

Several crimes were reported on a separate stretch of the trail earlier this summer and fall. Police said several juveniles have been arrested in a pair of attempted robberies in downtown Bethesda, a couple of miles north of Sunday’s incident.

No arrest has been made in a third crime, a strong-arm robbery, in the downtown section of the trail, police said.