A man accused of physically assaulting a woman at a U.S. research station in Antarctica was then sent to a remote icefield where he was tasked with protecting the safety of a professor and three young graduate students, and he remained there for a full week after a warrant for his arrest was issued, documents obtained by The Associated Press show.

Stephen Tyler Bieneman has pleaded not guilty to misdemeanor assault over the incident last November at McMurdo Station, which his lawyer said was nothing more than “horseplay.” The case is due to go to trial Monday in Honolulu.

The National Science Foundation declined to answer AP questions about why Bieneman was sent out into the field in a critical safety role while under investigation. The case raises further questions about decision-making in the U.S. Antarctic Program, which is already under scrutiny.

An AP investigation in August uncovered a pattern of women at McMurdo who said their claims of sexual harassment or assault were minimized by their employers, often leading to them or others being put in further danger.

And on Friday, the watchdog office overseeing the NSF said it was sending investigators to McMurdo this month as it expands its investigative mission to include crimes such as sexual assault and stalking.

In their indictment, prosecutors say that late on Nov. 24 or early Nov. 25 last year, a woman was sitting in a dormitory lounge waiting for her laundry when Bieneman, who had been celebrating his birthday with lots of drinks, walked in.

When he went to the bathroom, the woman took his name tag from his jacket as a prank and then refused to give it back, running around the end of a sofa, prosecutors say.

Bieneman then took her to the floor, put her on her back and put his left shin over her throat as he rummaged through her pocket looking for the tag, prosecutors say. The woman desperately tried to communicate she couldn't breathe, signaling a choking motion and tapping on his leg as a minute passed before Bieneman finally found the tag and removed his shin from her airway, according to the indictment.

Prosecutors say the woman visited a medical clinic.

“During a follow-up visit a week later, Victim A reported improvements with respect to muscle tightness, however she was suffering from lack of sleep and appetite, anxiousness, and depression as a result of the assault," prosecutors said in the indictment. ”Soon thereafter, Victim A left her employment at McMurdo Station."

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Bieneman's lawyer Birney Bervar said in an August email to the AP that eyewitnesses didn't back the woman's story and a doctor who examined her soon after the incident found no evidence of “an assault of the nature and degree she described.”

Marc Tunstall, the NSF station manager who is also a sworn Deputy U.S. Marshal, heard about the incident on Nov. 29 and began investigating, according to prosecutors.

On Dec. 10, two weeks after the incident, Bieneman and the scientific team flew by Twin Otter plane to set up camp at the remote Allan Hills icefield, more than 100 miles (160 kilometers) from McMurdo. The team, which studies ice cores, was there to collect radar data to help select a site for future ice-core drilling.

In his role as mountaineer, Bieneman was responsible for the safety of the group in the unforgiving environment. The man initially assigned the role had suffered from a mini-stroke two days before his deployment, according to documents obtained by the AP.

Bieneman, who goes by his middle name Tyler, initially worked well with the team setting up camp.

“However, soon after, it became clear that something was amiss with Tyler," University of Washington Professor Howard Conway wrote on behalf of the COLDEX field team in a complaint to the NSF that was obtained by the AP.

Conway and the graduate students did not respond to AP requests for comment.

In the complaint, Conway described Bieneman as initially being “domineering and critical” of the two female graduate students at the camp.

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“One evening in the kitchen tent during the first week, he told the graduate students that earlier in the season in McMurdo he had a fight with a woman, during which he wrestled with her, and she subsequently had trouble breathing, and needed medical attention," Conway wrote.

The professor said Bieneman portrayed himself as the victim in the incident for being under scrutiny. He said the graduate students, fearing possible retaliation if they disclosed the story, felt they had to tiptoe around Bieneman.

“It was uncomfortable and stressful to be around him because it was not possible to feel physically or emotionally safe," Conway wrote.

Court documents show an arrest warrant was issued for Bieneman on Dec. 12.

The professor wrote that Bieneman was finally replaced at the camp on Dec. 19. He said they were never told Bieneman was under investigation or given a reason for him being pulled from his assignment. They pieced it together later when the case became public.

“We were astounded to find (1) Tyler was assigned to our team when it was already known that he was under investigation, and (2) that he remained in the field with us for a full week after he had been charged with assault,” Conway wrote in the complaint.

The NSF said the questions about Bieneman's camp assignment were part of an active law enforcement matter and should be directed to the U.S. Attorney's Office in Hawaii. The U.S. Attorney's Office in Hawaii did not respond to a request for comment.

According to court records, when Bieneman returned to McMurdo after the camp, he was fired, given a plane ticket back to the U.S. and arrested when he landed in Hawaii. He was then released on $25,000 bail pending Monday's trial.