Convicted arsonist Thomas A. Sweatt in 2005. (Courtesy of WJLA News Channel 8./Courtesy of WJLA News Channel 8.)

In 2005, Thomas A. Sweatt admitted in court to setting 45 fires across the Washington region, claiming two lives.

He had only just begun to confess to his crimes.

On a car tour and in two days of interviews with investigators, the then-50-year-old fast-food manager directed agents to 309 additional fires he said he had started going back 20 years, including one on Quincy Place in Northwest Washington in 1985 that killed a couple and had initially been ruled an accident.

Sweatt’s agreement with prosecutors barred them from using those talks to charge him with more crimes, according to a federal law enforcement agent and a man whose father died in a blaze and was briefed by police. In exchange for his full accounting, Sweatt agreed to be imprisoned for life plus 136 years on a selected number of offenses he admitted to in court.

“We felt an obligation to the victims,” said Scott Fulkerson, who with his partner led the arson investigation for the Bureau of Alcohol, Tobacco, Firearms and Explosives and was explaining why it was important to get closure even if it meant forgoing additional charges.

Fulkerson, now in charge of the ATF office in Richmond, said his most difficult task was telling the four siblings who had escaped the blaze at Quincy Place that the flames that killed their parents, Bessie Mae Duncan and Roy R. Picott, were apparently started by Sweatt and not by a discarded cigarette, as the D.C. fire department had initially determined.

“It was not an easy day for us,” Fulkerson said. “It was a more difficult day for the family.”

The causes of the Jan. 11, 1985, fire that killed Duncan and Picott and of some of the other arsons ordinarily would have remained a secret between investigators and the families of those affected. But Sweatt has repeatedly boasted of his crimes in letters to a Washington City Paper reporter, and his case was showcased on television true-crime shows and in a book.

This past week, D.C. police revived the story by announcing that a detective assigned to solving cold cases had uncovered new details in the Quincy Place fire. The information led the D.C. medical examiner’s office, after 33 years, to reclassify the deaths of Duncan and Picott as homicides.

The police chief identified ­Sweatt as the prime suspect, although no charges have been filed, and none may ever be, given the age of the case and the hurdles of proving that the clues were not derived from the plea talks in 2005. Sweatt’s attorney with the Federal Public Defender’s Office did not return several calls seeking comment.

His attorney at the time he pleaded guilty said in court that his client’s mental illness meant he “did not have a choice” in setting the fires, that he was “laboring under a psychological compulsion.”

Sweatt, who later spoke with the media about the case, gave only brief remarks at his sentencing hearing. He apologized to the families of the victims, telling them, “I share your hurt,” and asking, for those who can’t forgive him, that “God replace the hate with understanding that time will heal.”

Picott’s son, Rodney Picott, a police officer in New York, said in a recent interview that investigators had told him in 2005 that Sweatt had confessed to setting the fire that killed his parents, and Picott accepted that it would not become part of the formal charges.

In an interview, Fulkerson said investigators reexamined photos from the Quincy Place fire after Sweatt told them he had started it. The fire, Fulkerson said, appeared to have started with lighted gasoline on the porch that seeped under the front door, and not with a dropped cigarette.

In a 2007 article, Sweatt told Washington City Paper that he had been walking home from his job that January day and passed ­Picott walking in the other direction. Picott said something, and the two nodded. Sweatt followed him home and then retrieved items to set a fire.

Vito Maggiolo, a spokesman for the D.C. fire department, remembers the fire well. At the time, he volunteered on the canteen, supplying firefighters with water, coffee and food at large fires. He had just returned to the station with the beverage truck when the Quincy Place fire call came out. He sped there in his own car.

Firefighters were inside the burning rowhouse searching for Duncan, who was trapped. The flames grew intense, and the firefighters were ordered to leave. As the last ones walked out, Maggiolo said, the cornice collapsed, engulfing them in dust and debris.

“I thought for sure we had lost some firefighters,” Maggiolo recalled. None died, but several were hurt. “I’ve been to thousands of fires, but there’s some that just stand out in your mind. That one is very vivid.”

Years later, Sweatt told City Paper that after his chance encounter with Picott on the street, he decided he liked him; he later confessed to fantasizing about his victims being caught in the flames. Duncan died in the fire, and Picott died weeks later at a hospital.

Fulkerson said Sweatt managed to stay under the radar for so many years because he didn’t fit the accepted profile of a serial arsonist — white, older and feeling wronged by society. In 2013, Fulkerson spoke to the International Association of Arson Investigators in Las Vegas and used Sweatt as a case study in solving tough crimes. Sweatt came under suspicion in 2003 after police linked shards of evidence from different fires — a strand of hair, a pair of pants, parts of the jug he used to set the fires. He was arrested in 2005.

Fulkerson said he told the investigators at the seminar, “You may have a serial arsonist in your jurisdiction and you don’t know it yet.” The ATF agent said he tells other investigators that suspects like Sweatt can be coaxed out of silence through feigned admiration.

“We had to say, ‘Look, we’ve been waiting to see you. We’re fascinated by the craft you created and the complexity and longevity that you’ve done fires,’” Fulkerson said. “We had to give him an opportunity to tell his story. Give him his time to shine. . . . As with most sociopaths, he likes to brag about what he did and how he did it.”

U.S. District Judge Deborah K. Chasanow appeared to understand that Sweatt’s sentencing hearing felt incomplete. Before she ordered him imprisoned for life, she noted that prosecutors had charged “selected offenses” that she said were “in no way a complete articulation of the crimes that were committed, nor the extent of the harm.”