Rescue workers called off their search about 8:30 p.m. for two men who witnesses said fell off their rafts in Sespe Creek and clung to rocks, crying for help.
But in five hours, divers and searchers in a helicopter found no one after exploring five miles of the mostly shallow creek near Fillmore.
"They could have crawled out downstream," said Ventura County Sheriff's Deputy Jim Wade. "We just don't know."
He said rescue workers will continue searching this morning.
Wade said three young girls who live nearby reported seeing the men in distress about 3:35 p.m. Less than a mile downstream from the girls, rescue workers found two rafts tied together and an ice chest full of beer.
About 20 rescue workers, made up of personnel from the Ventura County sheriff's and fire departments, along with volunteers from the county Search and Rescue team, concentrated their search near a rock overhang that formed a fast-moving current where the creek was about 6 feet deep.