Investigators are still piecing together what killed two private subcontractors near the largest US research station in Antarctica, but no criminal activity was immediately suspected, the National Science Foundation says.

The two fire-safety technicians died in unexplained circumstances on Wednesday while performing preventive maintenance on a building that houses a generator for a radio transmitter outside the NSF-managed McMurdo Station, the agency said on Thursday.

While the science foundation, a US government agency, said an official inquiry into the deaths was just getting under way, spokesman Peter West told Reuters that investigators had turned up no evidence of foul play.

He also said the deaths were believed to have resulted in all likelihood from some kind of accident or mishap.

West said he was not at liberty to disclose more about the investigation.

NSF also declined to disclose any personal information about the two workers, except to say they were employed by a Virginia-based company, PAE, which in turn was hired by the US Antarctica Program’s logistics contractor, Leidos, headquartered in Colorado.

PAE did not respond to requests for comment.

The science foundation initially reported the two technicians were found unconscious on the floor of the generator hut after a helicopter pilot flying over the area saw what appeared to be smoke coming from the structure and landed to investigate.

The NSF revised its account on Thursday, saying the pilot was on the ground a short distance away waiting for the pair to complete their work.

He walked up a hill to the generator to check on them when they failed to return to the helicopter landing site at the agreed-upon time.

One of the workers was pronounced dead by medical personnel called to the scene and the other a short time later at the McMurdo medical clinic, the NSF said.

The largest research outpost in Antarctica, the 60-year-old McMurdo Station lies at the tip of Ross Island in New Zealand-claimed territory called the Ross Dependency.

Daytime highs in December, in the middle of the Antarctic summer, average just below zero degrees.

Some 900 people, including scientists and support personnel, work at the station this time of year.

Human deaths are uncommon in Antarctica, despite its harsh environment.

As recently as October, a subcontractor died of natural causes at Palmer Station, one of two other NSF outposts on the frozen continent.

A 43-year-old electronic maintenance technician from Canada died at McMurdo on New Year’s Day 2000.

-AAP