South Haven, MI-

Investigators say gas fumes likely caused an explosion that sent the owner of a 44-foot yacht and three passengers on board the boat to the hospital, two with serious injuries.

But the source of those fumes and the reason they ignited remains under investigation.

The damage is evident. And boaters are still talking about what they saw Sunday afternoon.

“We were at North Beach, about an eighth of a mile away,” said witness Greg Danno. “We saw the black smoke. Like a plane crash.

Digital images show the disaster as it unfolded. The explosion put the four people near or onboard the boat into the water.

Michigan State Police say Jim Fish, the owner of the boat, is in critical condition at Bronson Hospital in Kalamazoo.

Three passengers from the Kalamazoo area, Keith Pepin, Anita Pryor and her son, Austin, were also injured. Pepin is in fair condition. Anita Pryor is in serious condition, while Austin Pryor was treated and released.

Investigators said they’re convinced the explosion was an accident.

They’re looking at the possibility that the boat’s owner was refueling the yacht just before the explosion, the fumes ignited by a spark on board.

Most boaters know to turn on the blowers, fans that blow any fumes out of the engine compartment, before starting the boat.

“Here, they weren’t looking to start the boat,” said South Haven Area Emergency Services Chief Ronald Wise. They were looking at probably trying to refuel it. It could have been something as simple as turning the gas caddy on, turning a switch on, doing something that, it could have been the vapors already on the boat.”

The chief said propane used for cooking could also be a source of the vapors.

But investigators won’t know for sure until they can get a closer look at the boat. Right now, it’s too unstable to climb aboard.

“Once we get the boat out of the water, the investigators will be able to tear through how everything is stacked back there, look underneath there and see if there are any clues,” said Chief Wise.