The Los Angeles County Sheriff's Department said it received an anonymous call around 1:30 a.m. Wednesday about a man who had been shot on Highway 39.
The man was shot twice in the stomach area. He was airlifted to L.A. County USC Medical Center in critical condition.
The man initially was found by passersby who were able to flag down some help.
"We were driving by, and we saw this guy that looked like a backpack on the side. We decided to turn around and check him out, and then we realized he was injured," said one man.
The passerby said he tried to call for help, but he didn't have reception on his phone so he flagged down another driver.
Authorities believe the shooter drove away in a black BMW.
Anyone with information was urged to call Los Angeles County sheriff's San Dimas station at (909) 450-2700.