The homeowner was hospitalized for a 72-hour psychological evaluation after being rescued from the house by a neighbor and then struggling with authorities who prevented him from re-entering, but he was not arrested.
Gary Eppler, who lives across the street, identified him as Thomas McKiernan, a retired machinist in his 60s who recently separated from his wife, has grown children and didn't seem unusual in the nine years he has known him.
Eppler said McKiernan had an interest in guns and has a son who is an Army sniper who went to Iraq last year and returned safely.
"I knew that he target-practiced and we'd talk about that. He'd go to a rifle range so I knew he had some ammunition," Eppler said.
McKiernan talked about reloading shell casings but said he hadn't done it in some time, Eppler said.
"I've been a psych nurse for 30 years and I didn't notice anything unusual," the neighbor said.
The day after the fire an incongruous scene played out in the windy hilltop neighborhood of older middle-class tract homes set against a backdrop of the snow-capped San Bernardino Mountains — part of a quiet semi-rural community that bills itself as "Horsetown USA."
Riverside County sheriff's deputies and federal Bureau of Alcohol, Tobacco, Firearms and Explosives agents carried charred boxes and pieces of furniture out of the house, stacked numerous boxes of ammunition in the driveway and examined seized rifles.
Two weapons were assault rifles that were illegal and the man had no permit for 75 pounds of black gunpowder that was found, said sheriff's Deputy Juan Zamora.
The tunnel, stretching from an entry in the garage 8-10 feet into the backyard, yielded three 25-gallon containers of fluid that was being analyzed by hazardous material experts, said Norco fire Battalion Chief Ron Knueven.
The tunnel was 5 feet wide and tall enough to allow someone to walk upright with room to spare, Zamora said.
Eppler said he was about to go on a bicycle ride Thursday afternoon when he noticed smoke coming from McKiernan's house. He said he alerted his wife, Linda, to call 911 and raced to his neighbor's home.
Inside, Eppler said, he found McKiernan on a couch.
"I looked and saw he was passed out on the couch and I woke him up and I had to help him up because he was staggering a little," Eppler said.
Eppler said it was smoky in the house but he didn't know if McKiernan had been affected by it.
McKiernan then ran to the back of the house, grabbed a hose and tried to pour water on his roof, Eppler said.
Eppler said that when firefighters arrived he convinced McKiernan to come to the front of the house but the man didn't think they were doing a good enough job because they weren't hosing down the roof, and he tried to re-enter the home. After McKiernan was restrained and put in a police car, ammunition began exploding, Eppler said.
The explosions forced evacuation of the area and kept firefighters at a distance. The blaze caused the roof to collapse before the fire was extinguished.
"It sounded like firecrackers, they were going off quite a bit," said neighbor Frank Jackson, who rushed home when he heard about the fire.
When he got there, he said firefighters were swarming over the burning house but the explosions were so intense that firefighters on the roof had to abandon it.
"The shells were going off and you had to back off," he said.
Authorities found a machine in the garage that was used to load the gunpowder into empty casings. The practice known as "reloading" is common and not illegal, ATF spokeswoman Susan Raichel said.
In 2003 McKiernan was quoted in a Riverside newspaper story about a fundraiser to build a military memorial in the city of Corona. McKiernan spoke to The Press-Enterprise about his brother Tim's 1968 death in the crash of a helicopter he was piloting in Vietnam, describing a heroic effort to avoid a school.
In 2002 he was in a newspaper photograph showing him hosing down his roof as a brush fire burned in the nearby Santa Ana River bottom.