A U.S. Forest Service ranger assigned to the east fork of San Gabriel Canyon above Azusa was assaulted Wednesday afternoon in a what authorities called a "bizarre and seemingly unprovoked attack."

The assault was reported about 3:40 p.m. after a man knocked on the door of the ranger station and asked to use the phone. When the ranger opened the door, he was struck in the face with an unknown object, the Los Angeles County Sheriff's Department said.

The blow knocked the ranger unconscious, Capt. Mike Parker said. The ranger was airlifted to a nearby hospital and was listed Wednesday evening in stable condition.

"We don't know know the reason for this bizarre and seemingly unprovoked attack," Parker said.

The suspect fled in a green Mitsubishi Eclipse.