Three New Brunswickers are in critical condition following a small plane crash in Alaska on Sunday.

Darrel Spencer, 66, of Fredericton, and his two sisters-in-law were about to embark on a cruise ship when they decided to take a sightseeing flight excursion through Antigun Pass in northern Alaska.

The four-seater Navion single-engine, low-wing plane went down in the remote area at about 1:45 p.m. local time.

Spencer underwent surgery on Wednesday. He is being treated for head trauma at a hospital in Anchorage, officials said.

His sisters-in-law, Marcene Nason, 65, and Daphne McCann, 57, were flown to Vancouver for treatment.

The pilot, Forest Kirst, was also injured in the crash and remains at the Providence Alaska Medical Center in fair condition.

Kirst, who is also the owner of Fairbanks-based Kirst Aviation, declined to comment on Wednesday.

An investigation into what caused the crash is ongoing, said Clint Johnson, chief of the National Transportation Safety Board's office of aviation safety in Alaska.

The plane crashed about 400 feet below the 4,700-foot summit in a very rocky, unstable area, said Johnson.

'We don’t know at this point right now if there were any mechanical issues because we haven’t had a chance to talk to the pilot.' - Clint Johnson, National Transportation Safety Board

"The wings, fuselage, pretty much all of the control surfaces sustained substantial damage," he said.

The weather and visibility were good at the time, said Johnson.

"We don’t know at this point right now if there were any mechanical issues because we haven’t had a chance to talk to the pilot," he said.

"There's still a number of pieces of the puzzle here that we don't have here at this point. Obviously being able to talk to the pilot is one a big one that our investigator is centering in on hopefully doing that either today or tomorrow."

The investigation could take up to nine months to conclude, Johnson said.

Investigators are currently working with the operator to remove the wreckage from the site and ship it to either Fairbanks or Anchorage for inspection.

Alyeska Pipeline Service Company contractors, who were working on a bridge in the area, witnessed the crash, alerted authorities and helped with the rescue, according to company spokesperson Michelle Egan.

"It's a pretty remote area, but we do have our own ambulances and medics," said Egan, who is the director of corporate communications.

"They’re really on-site for our own personnel, but in this case we were able to send them out to the site to help with the response."

Three ambulances and medics from each of the pump stations were sent, she said.

Atigun Pass is located at the head of the Dietrich River.