Sonoma County authorities are investigating a bizarre protest against a Berkeley nonprofit focused on rational thought after four people dressed in black robes and Guy Fawkes masks were arrested for allegedly barricading off a wooded retreat where the nonprofit group was holding an event.

The county sheriff’s office on Monday identified those detained at Westminster Woods in Occidental as Jack Lasota, 28, of Berkeley; Gwen Danielson, 25, a transient; Emma Borhanian, 28, of Albany; and Alexander Leatham, 24, whose city of residence is unknown.

All four were arrested late Friday on suspicion of felony child endangerment, false imprisonment and conspiracy as well as misdemeanor counts of resisting arrest, wearing a mask while committing a crime and trespassing.

Many aspects of the strange incident on Friday in Occidental remain a mystery, a sheriff’s spokeswoman, Misti Wood, told The Chronicle. The suspects were allegedly uncooperative and spoke incoherently, and Wood said investigators had to rely on fingerprints to identify them.

Lasota, Danielson and Leatham were jailed on $50,000 bail and will appear in court Tuesday. Borhanian posted bail Saturday and was released, Wood said. She will appear in court next week.

“They were on private property unwanted. It was frightening for everyone there,” Wood said. “We’re still putting it all together.”

Police determined the suspects targeted a group that had planned a weekend retreat at Westminster Woods. Flyers found by deputies suggested they were protesting artificial intelligence and the Center for Applied Rationality, a Berkeley nonprofit holding its annual alumni reunion weekend at the retreat’s conference center starting Friday evening.

“CFAR does not do remotely what they claim to do on their website: they do not appreciably develop novel rationality/mental tech,” the flyer claimed. “CFAR’s founding premise (that people were blocked in having the tools to think) was falsified long ago.”

The material pointed to an online blog that railed against the group as well as the Machine Intelligence Research Institute, another Berkeley nonprofit that does “foundational mathematical research to ensure smarter-than-human artificial intelligence has a positive impact.”

The Center for Applied Rationality issued a statement on Monday, saying it was familiar with some of the suspects: “Two of the individuals involved in this blockade had been asked not to attend our event as a result of prior erratic behavior towards our staff and others. Unfortunately, they instead took actions which disrupted things both for our guests and everyone else present at the camp. We were ultimately able to house our guests in an alternate venue on Friday night and then return to Westminster Woods once it reopened on Saturday afternoon, and our event proceeded on Saturday and Sunday without further incident.”

On its Facebook page, the Center for Applied Rationality posted: Friday afternoon, “four people wearing black robes and Guy Fawkes masks blocked the entrance to the alumni reunion venue in order to protest CFAR and MIRI, for reasons we don’t fully comprehend,” one organizer wrote. “The owners of the venue were understandably freaked out by this, and so called the police, who ordered the entire site evacuated.”

“From what I can tell, these actions seem like aggressive but non-violent protest theatre,” the organizer said. “If you feel worried, and no longer wish to attend the reunion, I think that is totally reasonable, and certainly do not blame you. ... We will not allow a precedent to be set that nonsense like this will achieve its disruptive aim.”

The incident began around 3:40 p.m. Friday at the entrance to Westminster Woods, a popular retreat with a conference center, ropes course and other activities along the Bohemian Highway in Occidental.

Police said the suspects arrived in a box truck, shuttle bus and Toyota Prius and used the vehicles to block the entrance and exit while dressed in black-hooded robes, black gloves, black silicone shoe covers and Guy Fawkes masks made popular by the movie “V for Vendetta” and the decentralized Anonymous hacktivist group.

Two of the individuals carried walkie-talkies, one wore a body camera and another carried pepper spray, officials said. Deputies found a written protest plan.

Wood said police received a report that one of the individuals might have a gun, which prompted a heavy response, including a SWAT team and a helicopter. No weapon was found, but the four protesters were physically and verbally combative with police officers at the scene and at sheriff’s headquarters, Wood said.

A fifth man was detained after claiming he was a journalist, Wood said, but he was released after deputies determined he had not committed a crime.

“The Sheriff’s Office supports people exercising their rights to peacefully protest,” Wood said. “These protesters were not peaceful, were arrested for multiple crimes, and caused fear and anxiety among the children and adults at the camp.”

A group of two dozen children were on the ropes course at the time of the four-hour incident — which shut down a stretch of the Bohemian Highway about 4 miles from the well-known Bohemian Grove — and were sheltered in an armored police vehicle.

About 50 people had to be evacuated from the property, Wood said, and driven to the Occidental Fire Department in a school bus. The bomb squad searched the vehicles for explosives and found none, Wood said.

Matthias Gafni is a San Francisco Chronicle staff writer. Email: matthias.gafni@sfchronicle.com Twitter: @mgafni