LAKE VIEW TERRACE (CNS) - Two people were critically injured Tuesday a third person suffered minor injuries as a thunderous explosion tore most of the roof from a commercial building and destroyed most of the 7,400- square foot facility in Lake View Terrace.

The explosion at 12349 Gladstone Ave. was reported at 4:20 p.m., according to Brian Humphrey of the Los Angeles Fire Department.

The explosion was heard from as far as a mile away and 911 calls flooded the LAFD, Humphrey said. Some people nearby were knocked off their feet by the blast.

The two critically injured victims were thrown from the building into an alley. All the injured, including a person who initially walked away from the explosion and then returned, asking to be taken to a hospital, worked in the building.

Humphrey said he could not imagine how someone from inside the building received only minor injuries, but that was the report on the person who left the scene for about 15 minutes. One of the critically injured was taken to Henry Mayo Newhall Memorial Hospital and the two others were taken to Providence Holy Cross Medical Center.

Humphrey said he did not know the name of the business, but said it was "an alternative fuel business involved in the extraction of hydrogen from water." He said the process involved two pressure vehicles with 20-inch diameters and as high as 8 feet tall.

Humphrey said it was not clear if the vessels were related to the explosion, but that is being investigated.

Humphrey told NBC4 that witnesses "described a thunderous explosion that sent a rolling steel door and other elements in this open business across the alleyway, throwing debris. We had people staggering from the scene and many others who reportedly were knocked off their feet in nearby businesses."

There was no fire, only an explosion, he said.

ABC7 reported that one victim lost an arm and part of a leg and the other had severe facial damage. Both were in critical condition.

Cuah Beltran, who works across the street from the building, told NBC4 he was "outside in the shop and all of a sudden we heard a loud explosion."

"You could actually feel it in your body," Beltran said. "It was, I mean it was tremendous. It was incredible. It sounded like a sonic boom. And all of a sudden we just see this big giant cloud of like metal, it looked like metal or foil up in the air, hundreds of feet up in the air.

"I made sure it wasn't in my shop, because it sounded like it was right inside my shop. Everybody ran out of the building."