A powerful blast and two-alarm blaze that tore through a Walnut Creek apartment building Friday morning, critically burning two people and causing its roof to collapse, resulted from a marijuana and hash oil drug extraction manufacturing process, police said.

"It was an incredible explosion, one I've never seen before," said Bob Grossman, who lives a block away from the six-unit building at 1564 Sunnyvale Avenue. "It shook our windows, and knocked over our windows."

The blast was reported just after 10 a.m. and sent two male burn victims to the hospital with critical burns, Contra Costa County Fire Protection Service Capt. Kent Kirby said.

Initially, Kirby said one person might be unaccounted, but he said later a search turned up nothing.

Officials said during the initial investigation after the blaze they found burned and exploded butane canisters, which clued them to a possible hash oil lab.

"We have strong evidence that what resulted in the fire, the injuries and the explosion was a butane hash oil extraction process," fire investigator Vic Massenkoff said. The volatile process uses butane as a solvent to "take out THC, the active ingredient in marijuana, into a very concentrated form."

“The serious danger is that, as they’re using this butane, raw butane vapors are spewing out into the room they’re in,” Massenkoff said. “Butane vapors are like gasoline vapors. They’re heavier than air; they sink to the ground and look for an ignition source. Something as simple as a spark of static electricity can ignite butane vapors.”

Less than 45 minutes after the powerful blast, the two-alarm fire was out, but Kirby said the two-story building was unstable and could collapse. Glass was shattered along the sidewalk for blocks.

Grossman told NBC Bay Area he helped rescue an older woman from a pile of debris.

"I saw the two burn victims and elderly woman yelling to get out of the lower apartment," Grossman said.

He and two others said they lifted away shattered wood and windows to get to her door with the fire right above her head before fire crews arrived.

"We were able to get her out of there," he said.

Bay City News contributed to this report.