Two civilian contract workers were killed and two others injured Wednesday morning while working on a barge at Pearl Harbor’s Naval Inactive Ship Maintenance Facility off Waipio peninsula.

Emergency Medical Services officials reported that four civilians were working on a floating concrete block when a 10,000-pound buoy fell about 70 feet, partially hitting two of them. The two others were injured while avoiding the buoy.

Healy Tibbitts Builders Inc., which specializes in marine construction, said the employees are theirs.

“We are devastated to learn of the loss of two members of the Healy Tibbitts family,” Rick Heltzel, company president, said in a statement. “Our thoughts and prayers are with the families at this extremely difficult time. We do not yet know what caused the incident and will be working with the Navy and other agencies to investigate this tragic event.”

The Navy also issued a statement expressing condolences to the families and friends of the victims. “A full investigation into the incident is underway, and we will work closely with local officials and agencies,” Navy officials said.

The workers were strengthening mooring lines on the 820-foot decommissioned amphibious ship Tarawa, which uses a number of the white, circular buoys, an official said.

The industrial accident was reported at 10:19 a.m. on a barge with a crane.

One of the civilian contractors who died was a male in his 40s and another who was reported in critical condition earlier in the day was in his 30s. The two others, a 50-year-old man and a 27-year-old man, were in stable condition.

Honolulu Fire Department spokesman Capt. David Jenkins said the men were on board a barge that was servicing the Navy’s fleet of inactive ships.

Two of the men were knocked or jumped into the water to avoid the buoy, which was 12-to-15 feet in diameter. One man reported jumping 15 feet from the concrete barge into the water.

Jenkins says the firefighters received a 911 call at 10:19 a.m. and arrived at the scene six minutes later. The contractors were still on the barge so firefighters didn’t reach them until 10:39 a.m. 

The facility is owned by the Navy as a holding area for decommissioned naval vessels, pending determination of their final fate and is part of the  Naval Sea Systems Command’s Inactive Ships Management Office based in Portsmouth, Va.

Naval Sea Systems had 13 ships in the Pearl Harbor’s Middle Loch in 2013.

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The Associated Press contributed to this report.