Two crew members of a tugboat were killed and two others were injured in what the U.S. Coast Guard described March 18 as a “confined space incident” aboard a barge moored in Southeast Alaska.

A Coast Guard news release provided limited details about what happened to the four crew members, but said they were in a confined space aboard the freight barge Waynehoe on March 15 when other crew members from their tug, the Chukchi Sea, lost contact with them. The barge was moored about 25 miles northwest of Ketchikan.

The parents of Sidney Mohorovich, one of the victims, said they were told by Coast Guard officials there was methane gas present in the confined space.

“We don’t know why the series of events that led to all the people being in the confined space, if they all like went down as a team or in separate stages,” Todd Mohorovich told The Associated Press by phone from his home in Sedro-Woolley, Washington.

“I have no information on that, but what I can tell you is that the confined space had high levels of methane gas in it.”

He did not know the source of the gas. The Coast Guard did not respond to an email seeking confirmation of the presence of methane gas.

U.S. Coast Guard Capt. Stanley Fields, commander of Sector Southeast Alaska, said the Coast Guard is conducting a thorough investigation into the accident.

“This is a heartbreaking reminder that confined spaces on vessels can contain extremely dangerous, invisible hazards,” he told the Daily Sitka Sentinel. “The Coast Guard is committed to a thorough investigation to understand what happened and prevent a tragedy like this from occurring again.”

Todd and Eva Mohorovich last spoke to their son the night before the accident, when he told them about impending bad weather. “He said that the barge was in a spot where they were going to be able to be sheltered from that storm,” Todd Mohorovich said.

The crew planned to perform normal deck duties to make sure everything was secured ahead of the storm.

Federal regulations define “confined space” on a vessel as “a compartment of small size and limited access such as a double-bottom tank ... or other space which by its small size and confined nature can readily create or aggravate a hazardous exposure.” That could include a lack of oxygen.

The tugboat crew called the Coast Guard for help and managed to recover one of the dead crew members from the space and helped the two survivors out before the rescue team arrived.

The barge was then towed to Ketchikan, where the confined space “was able to be safely cleared for the recovery of the second deceased crew member,” Coast Guard spokesperson Alexander Ransom said in an email.

The causes of death were not released, and the bodies were sent to Anchorage for autopsies.

The other man killed was identified as Ben Fowler, of Sitka.

The 92-foot-long tug also returned to Ketchikan last week. It is owned by Hamilton Marine Construction, of Bellingham, Washington.

Sidney Mohorovich, 28, was one month into his new job with Hamilton Marine. The company didn’t return a message seeking comment.

Mohorovich, a large equipment mechanic, was on his first job in Alaska. He lived in Deming, Washington, with his fiancée ahead of their planned June wedding.

He previously was a logger and welder, and before that he learned how to build houses and do electrical work. “He could pretty much figure anything out,” his mother said.

The two survivors were reported to be in good condition, the Coast Guard reported.