Sheriff's Lt. Dennis O'Leary says the bodies of the men were found around 3:15 p.m. Tuesday when a brother of one of the men and his girlfriend went to the home near Forestville to check on them.
O'Leary says detectives were interviewing family members and friends of the men late Tuesday.
The names of the victims have not been released, and investigators have not said if they have identified any suspects.
The property where the killings took place is near Iron Horse Vineyards. Winery employees said late Tuesday they've been told not to go near the area because of the law enforcement activity.