The man found lying unconscious with head wounds on Capell Valley Road/Highway 128 in the early hours of Monday morning has been identified as a 42-year-old Vacaville man.
Dustin Wayne Valentine remains in critical condition, said Napa County Sheriff's Capt. Tracey Stuart.
Valentine was spotted in the eastbound lane by a passing motorist shortly after midnight Monday and was flown to John Muir Medical Center in Walnut Creek for treatment.
CHP initially reported that Valentine was the victim of a hit and run, but sheriff's investigators said his injuries did not suggest he had been struck by a vehicle.
Stuart asks anyone with information to call detectives at (707) 253-4591.
She said people may also make anonymous tips through Crime Stoppers by texting NVCS and a message to 274637 (CRIMES) or online at https://www.tipsubmit.com/WebTips.aspx?AgencyID=912