BETHEL - A chain reaction of unusual circumstances touched off a propane gas explosion, shock wave and fire on Friday night that obliterated three of 12 Eden Ridge condominium units off Vernon Street.

Three other units in the two side-by-side buildings were damaged. There are six tri-level townhouse-style condo units in each building, which were built in 1987.

The units were a privately-owned mix of mostly second homes, Mahoosuc Realty broker Matthew Hiebert of Bethel said on Saturday. Some were rented.

Damage was estimated in the hundreds of thousands of dollars based on the units' average sale price of $180,000 to $200,000, Hiebert said.

Of four people home at the time of the 9:10 p.m. blast, three were hurt: Stephen and Ellen Little, 33 Walpole St., Dover, Mass., and Marvin Collins. Each lived on either side of the blast in unit No. 5. They suffered shrapnel wounds, officials said Saturday morning.

"There very easily could have been people killed," State Fire Marshal's Office investigator Sgt. Ken Grimes said. "Units across the way are half full, so there should have been 20 people in these buildings. Fortunately, no one else was here."

The Littles, who are in their 80s, suffered cuts and bruises from flying glass and debris that was blasted through their wall by a pressure wave, Grimes said. They were treated at the scene.

Collins, a Bethel and Newry firefighter, is also vice president of mountain operations at Sunday River Ski Resort in nearby Newry.

Bethel police Lt. Shayne R. White - the first to arrive - found Collins on his hands and knees in the doorway of his burning home and pulled him outside, fire Chief Mike Jodrey said. Collins was taken to Stephens Memorial Hospital in Norway.

After being treated and released, Collins returned to work at the ski area Saturday morning, but later left, officials said.

Grimes said the blast originated in the basement of condo No. 5. Snow had recently been shoveled off roofs behind the units, leaving a large pile.

"We believe that a large chunk of ice fell off the roof and hit the pile, and slid down between the pile and building and sheared off the main propane line regulator," Grimes said.

Propane leaked from one or more of three 500-pound underground propane tanks into the No. 5 unit through dryer vents and fittings around pipes until the vapors were ignited by a boiler or other spark in the basement, Grimes said.

Jodrey said the blast rattled his Vernon Street house a mile away.

"I heard a real deep-throated whoom; just one explosion," he said. "I didn't know what it was at first, then, a couple of minutes later we got toned out for an explosion."

"Everybody in the whole neighborhood said they thought their roofs fell in," firefighter Jeff Angevine said. "It was one big bang. I heard it in Newry and I thought it was fireworks going off at Sunday River."

"It sounded like a loud bang that was similar to a roof caving in, and shortly after, there was smoke in the air and the sky was orange, and I went to investigate. It was pretty unbelievable. Flames were 20 feet high at (9:45 p.m.) and smoke was pouring out of the roof," Hiebert said.

Larry Wolbarsht of Manchester, N.H., who lives beside the Littles, said the shock wave's "big mounds of air" nearly knocked him off his couch. He said he thought a car had exploded and went outside to check.

"I started to go next door and when I stepped out, I saw the building was gone and that answered my question of what happened," Wolbarsht said Saturday. "It was quite an experience. I just couldn't believe it. Friends of mine would have been in (No. 5), but they didn't come up yesterday. There was a pile of rubble and debris everywhere and shortly after, it caught fire."

Jodrey, who was seconds behind officer White, said that when they arrived, two condo units were gone and the road was full of debris that had to be snowplowed to allow firefighters access.

He called in about 40 firefighters and firetrucks from Andover, Greenwood, Newry and Woodstock. Several ambulances also responded.

Grimes said blast debris was thrown 250 yards over the hill behind the condos and 250 to 300 feet forward over another row of condos and into Vernon Street.

"It was quite powerful to move debris as far away as it did," Grimes said.

Heat from the resulting fire could be felt on Vernon Street, realty broker Hiebert said.

It took hours for firefighters to reach two of the three propane tanks to shut them off because they were buried by rubble, Jodrey said.

"The propane tanks were venting into the building. ... It literally incinerated (Collins's) Honda car," Jodrey said.

It was 3:30 a.m. Saturday before fires were out and firefighters started chasing hot spots. Hours later, smoke rose in several places while four of Maine's 10 fire marshal's investigators worked the scene. They were assisted by two state police accident reconstruction experts.