Investigators are looking into whether a blast at a home in San Diego’s Valencia Park area stemmed from an at-home explosives making operation, and not a methamphetamine lab as was first suspected.

The DEA Narcotics Task Force said the explosion that erupted around 4:30 p.m. Friday at a home on South Valencia Parkway stemmed from a small meth lab.

Investigators with the San Diego Metro Arson Strike Team (MAST), DEA, and ATF, some with Weapons of Mass Destruction training, spent the weekend detonating and removing dangerous chemicals from the property.

Investigators said chemicals found at the home were consistent with chemicals used to make meth and explosives. MAST, the agency leading the investigation, told NBC 7 Monday the investigation has shifted focus from meth to bomb making.

Homeowner Darell Roasa, 38, is being treated at UC San Diego Medical Center for burn and shrapnel injuries suffered in the explosion.

Detectives believe something exploded in his hand, and that object had shrapnel in it, which hit his face, arms and upper body.

Roasa told NBC 7 Investigates over the phone that he wasn't making meth or bombs.

"I am into experiments, making masonry tools that drill through concrete. That's what I was doing when the incident happened. I can't believe they are saying it's a meth lab. That's really mind blowing," he said.

Roasa has not been charged with a crime and it has not been determined whether the chemicals found were even illegal.

NBC 7 spoke with neighbors Saturday who said they’ve been hearing a series of loud noises and booms coming from the home over the past few days. The house is located across the street from an elementary school.

Federal agents confirmed Roasa had an underground bunker in his backyard, but they did not verify what it was used for. Investigators did, however, use the bunker to destroy volatile chemicals that couldn't be safely removed from the property.

Roasa also had a shed filled with used barrels, appliances and other materials that had the look of a makeshift lab.

Investigators say all of the dangerous chemicals have been removed and Roasa's wife and three children have been allowed to return home.

As for the weapons found at the home, Roasa says they were unloaded and legally purchased.

NBC 7 Investigates has been reporting on the recent rise of methamphetamine in San Diego County. In the late 1980s, the county was a hotbed for meth production inside makeshift home labs.

Over the past few years, there has been a shift in the production of meth – from backyard labs to intricate super labs controlled by Mexican drug cartels.

Editor's Note: A previous version of this story said MAST investigators saw evidence of a bomb lab. A spokesperson for the unit clarifies that MAST is investigating explosives but not necessarily bomb-making.