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A father and son are accused of turning a routine wildlife research trip into a terrifying 15-hour hostage ordeal after allegedly abducting two US Forest Service biologists at gunpoint in a remote Northern California forest, federal authorities said.

Joseph Charles Henrichsen, 49, and his son, Phoenix Henrichsen, 23, were charged Monday with kidnapping federal employees after authorities say they held the women captive inside a camping trailer near Gumboot Lake Campground in the Shasta-Trinity National Forest.

The biologists were conducting a Forest Service study of Northern California frog habitats on Thursday when Joseph Henrichsen allegedly confronted them with an AR-15-style rifle, ordered them to raise their hands and zip-tied their wrists behind their backs, according to an FBI affidavit.

Investigators said the women were then forced into a nearby white camping trailer, where Phoenix allegedly helped clear space for them while they remained restrained.

One of the biologists later told investigators Phoenix stayed behind to watch over her whenever his father left the trailer.

During the ordeal, Joseph Henrichsen allegedly drove one of the women several miles toward Lake Siskiyou in the Forest Service’s Nissan Frontier while armed.

Investigators say he used her cellphone to leave a voicemail for an acquaintance and call a Forest Service ranger.

“I’ve taken two fed biologists hostage from the Forest Service,” Henrichsen allegedly said.

According to the affidavit, he also said the woman was zip-tied with her hands behind her back and warned he would use live ammunition against anyone who tried to intervene.

Authorities were alerted Thursday morning, and the FBI began negotiating with Henrichsen later that afternoon after he contacted officers over the Forest Service vehicle’s radio while identifying himself as “Charles Perry,” an alias listed in the federal complaint.

“I have two biologists in my custody right now,” Henrichsen said in archived radio traffic. “… I’m requesting an FBI investigator to speak to so that I can release these hostages.”

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“I have two biologists in my custody right now,” Henrichsen said in archived radio traffic. “… I’m requesting an FBI investigator to speak to so that I can release these hostages.”

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Negotiations continued for roughly nine hours as federal, state and local law enforcement surrounded the remote campground.

Investigators said the standoff escalated when Joseph Henrichsen allegedly claimed he had a grenade and threatened to use it unless officers moved away from a nearby truck.

Shortly after 1 a.m. Friday, one of the biologists was released and walked to waiting officers. The second woman emerged about six minutes later.

Both women later told investigators they had been kidnapped at gunpoint by Joseph Henrichsen and held against their will.