The man wrote a suicide note and then shot himself in the head Wednesday night at his home near Mayhill, Otero County Sheriff John Lee said.
"He felt that he couldn't live with what happened," Lee said.
The sheriff did not release the contents of the letter, but he said the man didn't know whether it was a cigarette or a spark from his vehicle that started the fire on Tuesday. "He did know he caused it," Lee said.
Lee said authorities had questioned the man about the fire several times but didn't have enough evidence to arrest him.
The fire burned a swath through southern New Mexico's mountains on Wednesday, jumping from 800 acres to 9,500 acres in a day as winds gusted up to 60 mph.
Authorities could not estimate how many people were evacuated from the rural area. The governor's office declared a state of emergency Thursday morning for the area.
"Now we have another tragedy on top of a tragedy," Lt. Gov. Walter Bradley said. "The fact is, we know this gentleman. He lived in Wills Canyon. He was a very honorable and honest man and he just felt he could not face his family and friends with what happened."
Crews were sent to New Mexico from a largely contained blaze in southern Arizona that charred 36,000 acres of dry grass and oak brush
Rick Hartigan of the U.S. Forest Service said windy conditions and erratic fire movement made Wednesday "a horrendous day," as the New Mexico fire ballooned and slurry planes were grounded because of wind.
Forestry officials said the 20 structures that burned were west of Mayhill, a town of about 250 people 100 miles north of El Paso, Texas. They said at least two were homes.
Fifteen minutes after being advised to leave, 79-year-old Mayhill resident Hazel Perry grabbed some clothes, makeup and photos, including one of her father when he was serving in World War I.
"The fire was licking up over the top of the mountain there," Perry said. She feared her home was gone. "I just can't bring myself to accept it."
David Williams, who lives southwest of Mayhill, had bloodshot eyes and a red face after he and 18 firefighters took cover in a burned out area near his home as the fire raged out of control.
"I knew for a fact this was the one I had been expecting for 10 years," Williams said. "We thought we had it under control (Wednesday) morning. Then it exploded, a real firestorm."
Decreased wind in Arizona allowed firefighters to make headway against the blaze there, which was 75 percent contained Thursday morning. Full containment was anticipated by Thursday night.
The blaze burned one home and two barns but firefighters saved up to 100 buildings. No injuries were reported.
The fire also damaged power lines, knocking out power to Fort Huachuca for about 15 hours before it was restored Wednesday afternoon.
The fire, which began Monday in the Coronado National Forest, stalled about 1½ miles from Huachuca City. The cause of the fire was unknown.
In southern Colorado, a 600-acre wildfire in Custer County burned two houses, but was fully contained Wednesday night.