BOISE - Two men in their 20s are at the Salt Lake City burn center for treatment after they were injured when a home exploded this morning in Boise's North End.

Police say one of the men hurt in the explosion received fairly serious injuries. Two men received burns to the upper bodies, mainly hands, arms and faces. .

It happened just after 4 a.m. Monday on the 1600 block of North 6th Street in north Boise.

Four people were in the home at the time and say they smelled gas before the explosion.

When crews arrived they didn't find any flames but did say there was damage done on the inside and outside of the home.

Structural collapse experts have spent hours trying to shore up the front of the house and it is nothing compared to the back of the house.

"Actually it's extremely unusual, in fact in anyone's recent memory they can't remember an explosion like this where they were still scratching their heads for a cause, hours later," said Boise Police spokeswoman Lynn Hightower.

We talked with neighbors who described what they heard and saw.

"The whole back of the house is destroyed, the roof came down and all the windows got blown out," said Patrick Miller.

"Just a big explosion and it rocked our whole house," said Lois Santillanes who lives next door.

The blast was so strong it also knocked the clock off of Santillanes' wall.

"The battery literally flew out of the clock and we found it by the door," she said.

"What appears that happened is they had an explosion in the rear of the structure. Not sure what caused the explosion yet," said Boise Fire Battalion Chief Jake Ellis.

Four people were in the house when it exploded. Lois says shortly after she could her one of her neighbors screaming.

Boise Police say earlier that night the men woke up to an odd smell. They were in the utility room when the explosion occurred.

Another man and woman were in the front of the home and were not hurt.

In the daylight you could see just how bad the damage was.

"It's just amazing!" said Santillanes.

And amazing everyone in the house go out alive.

Fire and police investigators are delaying entry to the home because of safety concerns. An engineer will inspect the home first for the possibility of further collapse.

At this time, they tell us they are still looking into the cause of the explosion, including the possibility of a gas leak.

The blast scared the family dog which ran off toward the Boise Foothills. The dog was found safe around 11 a.m. and has been returned to its owners.

Boise police and fire investigators have determined that an accidental natural gas leak is the cause of the explosion that occurred shortly after 4 a.m. Monday at a home on 6th Street.

Boise Police spokeswoman Lynn Hightower tells KTVB that the leak happened when a valve came open on an unused gas line that had been capped.

Two men have been taken to the burn center in Salt Lake City.