Two biologists with the US Forest Service were surveying frog habitats in the remote wilderness of the Shasta-Trinity national forest in north-eastern California late last week when a man allegedly approached them carrying an AR-15.

The man ordered the women to turn around with their hands in the air, and then, according to court documents, used zip ties to restrain them before ordering the pair into a nearby trailer where authorities say they were held hostage for roughly 15 hours.

The incident, which prompted a massive response from law enforcement across the region including local sheriffs offices, police departments and the FBI’s hostage response team, was described in court documents published earlier this week.

The criminal complaint filed in federal court offers the most detailed look yet at the events that unfolded on 16 July into the early morning hours the next day.

However, there is still little that is publicly known about the motivation for the crime.

Authorities say Joseph Charles Henrichsen, 49, and his 23-year-old son Phoenix Henrichsen kidnapped the federal employees.

The elder Henrichsen allegedly first confronted the biologists, who have not been publicly identified, while his son cleared space for the women in a trailer where they had been staying near Gumboot Lake in Siskiyou county. At one point, the 49-year-old separated the hostages and drove one to a nearby lake, where he used her phone to call a USFS ranger and also left a voicemail for a person he knew.

In the message, Joseph allegedly said: “I’ve taken two fed biologists hostage from the forest service. [Victim 1] is ziptied, her hands are behind her fucking back, and I’ve got live rounds ready to fucking [inaudible] anyone who fucks with me,” according to the complaint.

“So what I’d like to have happen, I’d like to have you send me a fucking fed investigator, with a goddamn badge, preferabl[y] from the FBI. And we can talk things through a little bit.”

Jeremiah LaRue, the Siskiyou county sheriff, told the Sacramento Bee his office was initially unsure whether the call was real until a drone captured footage of the campground showing an armed man and a restrained hostage.

There was a “rapid buildup” of law enforcement around the lake in response to the incident, according to the complaint and authorities began negotiations with Joseph that afternoon. The discussions stretched over several hours with little progress, and in the evening Joseph allegedly told law enforcement officers he had a grenade and would detonate it if authorities that had been approaching did not retreat from the scene.

After hours of negotiations, Joseph agreed to release one of the hostages, according to the complaint, and several minutes later the other left the trailer as well.

LaRue was unsure whether the father and son made specific demands, but told the Sacramento Bee that “[Joseph] wanted to talk to the FBI about ‘what’s going on in the world’, I believe is the most specific information”.

Joseph and Phoenix Henrichsen surrendered without incident shortly about 2.30am. They were charged with two counts of kidnapping a federal employee.

The father and son previously lived in Washington. In 2022, Joseph had been accused of a hate crime for allegedly harassing his landlords, the Bellingham Herald reported. A judge ruled Henrichsen incompetent to stand trial but the case was dismissed due to delays in admitting him to a state hospital.

Phoenix at the time described his father to authorities as “unstable” and “prone to emotional outburst”, the Bee reported.

The FBI has said it is seeking “first-hand information” from the public to aid in its investigation of the case. The father and son face a maximum sentence of life in prison and a $250,000 fine if convicted.