Investigators first learned of the farm after the man died at Enumclaw Community Hospital July 2. The county Medical Examiner's Office ruled that the death was accidental and the result of having sex with a horse.
A surveillance camera picked up the license plate of the car that dropped the man off at the hospital, which led detectives to the farm and other people involved, said sheriff's Sgt. John Urquhart.
Deputies don't believe a crime occurred because bestiality is not illegal in Washington state and the horse was uninjured, said Urquhart.
But because investigators found chickens, goats and sheep on the property, they are looking into whether animal cruelty — which is a crime — was committed by having sex with these smaller, weaker animals, he said.
The farm was talked about in Internet chat rooms as a destination for people looking to have sex with livestock, he said.
"A significant number of people, we believe, have likely visited this farm," said Urquhart.
The Humane Society of the United States intends to use the case during the next state legislative session as an example of why sex with animals should be outlawed in Washington, said Bob Reder, a Humane Society regional director in Seattle.
"This and a few other cases that we have will allow us a platform to talk about sex abuse of animals," Reder said.
Thirty-three states ban sex with animals, he said.