St. Paul firefighter Joaquin Rosales and several of his colleagues rushed to a pile of rubble on the city’s East Side Friday morning when an unusually large explosion destroyed a home and damaged several buildings.

They were searching amid piles of debris and exposed nails when Rosales heard moaning and groaning.

“You could hear him calling out,” Rosales recounted Monday afternoon. “Immediately I just began doing my job. I’ve never experienced anything like this.”

Rosales was the first person to discover John Lundahl, 80, buried under insulation and debris in the 600 block of Payne Avenue. Rosales called out to his colleagues, and with the help of about four other firefighters, pulled Lundahl out as several spot fires burned around them.

Lundahl was taken to Regions Hospital, where he remained in critical condition Monday.

“I think the actions that they took were extremely heroic,” Assistant Fire Chief Matt Simpson said at a news conference Monday afternoon.

The cause of the explosion remains under investigation, fire officials said.

“I would expect this one to take a little longer,” St. Paul Assistant Fire Chief Mike Gaede said of the investigation. “Nothing’s been ruled out.”

Gaede said there were no updates about the possible cause.

Deputy St. Paul Fire Chief Dave Berger said Friday that a gas leak could be responsible, but Gaede declined to speculate Monday.

“That’s the initial assumption,” Berger said Friday. “With something of this magnitude, the first thing we think of looking at is a gas explosion. There are few other things that it could be.”

Xcel Energy provides gas and electric service to the block where the explosion occurred. An Xcel spokesman declined to comment Monday, deferring to the fire department.

Simpson said he was unaware of any digging activity at the home that could have affected gas lines. Angie Wiese, the city’s fire safety manager, said there was no work being done in the immediate area by the city’s public works department or Xcel Energy.

Simpson said he doesn’t believe investigators have had an opportunity to speak with Lundahl due to his injuries. The investigation could take weeks, Wiese said.

While the fire department is investigating the explosion’s cause, the Minnesota Office of Pipeline Safety will examine — if the explosion is deemed a gas leak — whether any state regulations were broken. The pipeline office, a division of the Minnesota Department of Public Safety, can fine companies for violations.

Fire Capt. Dennis Hall, who arrived at the scene early, said he has seen about five building explosions in his 25 years with the department.

“This one was very major,” Hall said. “It happens. Thankfully it’s very infrequent.”

Fire Capt. Jeremie Baker, who was also at the scene, said it was “miraculous” that Lundahl survived the explosion.

The force of the explosion was so strong it damaged a building as far as a block away, Wiese said.

According to Wiese: Six buildings, including Lundahl’s home, were condemned. Homeowners are responsible for repairing the damage, and the city has authority to allow residents to move back into the buildings. Fourteen other buildings, including homes and garages, had damage such as broken windows and cracked walls and ceilings.

The exact number of people who were displaced is unclear, Wiese said, but the American Red Cross helped find temporary housing for 13 adults and three children. They remained displaced as of Monday, and it’s unclear when they will be able to return home.

“It could have been so much worse,” Wiese said, giving the damage an 8 on a scale of 10.

The St. Paul Fire Foundation, a nonprofit organization, is raising money to help those who were displaced by the explosion.