A 51-year-old man who is accused of repeatedly threatening that this place “is going to burn” has been arrested and charged with starting the Holy Fire, causing nearly 10,000 acres to be set ablaze and more than 20,000 people to be placed under evacuation orders at Cleveland National Forest in Southern California.

The arrest was made as more than 18 fires across the state continue to endanger the lives of thousands, including hundreds of firefighters and first responders working around the clock. One, the Mendocino Complex Fire in Northern California, is the largest wildfire in California history. Yosemite National Park has been closed “indefinitely” because of smoke from another fire.

The Bay Area Air Quality Management District has issued a Spare the Air guideline through Thursday night, saying that children and the elderly should stay inside and minimize direct exposure because of smoke and particle matter from Northern California wildfires.

The Holy Fire, which started in the Holy Jim Canyon area, has been burning in Southern California since Monday, with more than 600 firefighters assigned to fight it. Two firefighters have been treated for heat-related injuries. The fire remains only 5 percent contained.

Mike Milligan, 71, who calls himself a volunteer fire firefighter and chief, said in an interview with The Washington Post that he has been flagging problems with suspect Forrest Gordon Clark for more than three years. He said he alerted the U.S. Forest Service that “you have to do something or he’s going to kill someone or burn this place down.”

Milligan said he received several texts from Clark last week threatening to start a fire.

“In a text, he said this place ‘is going to burn just like we planned,’ ” Milligan said, adding that he reported it to the sheriff’s office and again to the U.S. Forest Service. “Why the hell didn’t they respond? I reported this over and over again.”

Milligan said he’s known Clark for 10 years and has been concerned about his mental health and behavior in the tiny mountainous area, with only 17 recreational cabins at the bottom of a steep cannon.

The area is very rustic and reclusive, and tends to draw “unique and quirky individuals who enjoy living by themselves in a remote area, with limited personal interaction,” said Olivia Walker, public affairs officer for Cleveland National Forest, a local branch with the U.S. Forest Service.

But it has hiking trails, lush trees and a waterfall, one of the few in Orange County. There’s no country store there and only hikers pass through occasionally, Walker said.

Clark had been involved in volatile disputes with neighbors — on their rented recreation forest cabins — for years and appeared to believe in conspiracy theories, Milligan said. The land is owned by the federal government and the cabins are supposed to be part-time residences.

Walker said she wasn’t aware of Milligan’s reports of concern. Local police did not respond to requests for comment.

“Even if someone here had been aware of odd behavior, there was nothing we could have done unless the person was violent- that would fall under law enforcement,” said Walker. “You can come here and act the fool all you want, but please don’t start fires. It’s a big loss to the area — scenically it’s a big loss.”

“This has really impacted people’s lives and the number of evacuees is only going to keep rising,” Walker said. “Things like heat and wind — the fire has room to grow and burn. We are hoping this weekend we will get cooler weather so the firefighters can really get in there and actively engage it instead of taking a more defensive stance. It’s just very sad.”

Charges against Clark include: suspicion of aggravated arson with the intent to cause injury to another or damage property; arson of an inhabited property; arson of forest; making criminal threats and resisting arrest. He faces an additional count of arson that caused multiple structures to burn.

Orange County Supervisor Todd Spitzer told The Post that he hadn’t been aware of the threatening texts allegedly sent by Clark.

“The community is angry and sad. They live with fire threat nearly year-round so an intentional act is mind-numbing. Anyone who started this fire is a monster and should serve life in prison,” Spitzer said in a phone interview, where he cited excessive heat warnings. “Who would go out with low humidity, and high wind and the highest heat temperatures this time of year, and intentionally set the forest on fire?”

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King Bass, 6, left, and his sister Princess, 5, watch the Holy Fire burn while sitting on the roof of their parents car in Lake Elsinore. (Patrick Record/AP)

The air quality was so poor, Spitzer said, that “you can taste it — like tin metal — in the back of your throat.” The Air Quality Management District put out a warning Thursday, he said. “The whole state needs more resources. This is becoming the rule, not the exception, and it’s sad to watch.”

Shane Sherwood with the Orange County Fire Authority told reporters that he could not comment on the alleged text, saying the investigation was ongoing. But he said the evidence gathered led investigators to believe the fire was “intentional.”

He said they were still working through the evidence to determine how it was started.

Sherwood said that no witnesses have come forward saying they saw Clark start the fire but that the area of origin is around his residence.

Clark, who is being held on $1 million bond, refused to leave his cell to appear in court. Before he was arrested, Clark told a cameraman that he was asleep and had no clue how it started and that “gangbangers” are out to kill him. It was not clear who Clark’s legal counsel is.

Milligan told the Orange County Register that Clark had “run through the community screaming” and claimed he could read minds. On Tuesday, he stripped off his camouflage underwear in public, the paper reported.

The paper also said a man with the same name and birth date was held involuntarily in 1996 at a treatment center for mental illness.