More than three decades after investigators concluded a fire that killed a couple inside their Eckington rowhouse was an accident caused by a carelessly discarded cigarette, D.C. police said Monday the deaths have now been ruled homicides.

While no arrest was announced in the deaths of Bessie Mae Duncan, who was in her 40s, and her husband, Roy R. Picott, 39, authorities said they are focusing on a serial arsonist, Thomas Sweatt, who admitted setting a string of 45 fires in the D.C. region and pleaded guilty in 2005.

Sweatt, now 63, is serving a federal prison term of life with an additional 136 years. Washington City Paper reported in 2007 that Sweatt confessed in a series of letters to a reporter to setting fires for years longer than covered in his guilty plea, including the one that killed the Eckington couple. The case was the focus of television crime shows on truTV, Investigation Discovery Channels and as part of the Forensic Files series.

D.C. Police Chief Peter Newsham said Sweatt is the prime focus of the investigation. He has not been charged in the deaths of Picott and Duncan, and authorities said prosecuting a decades-old case could be difficult. Newsham said a detective in the homicide unit’s cold-case squad revived the inquiry that led to the manner of death being changed from accidental to homicide. The fire occurred Jan. 11, 1985, in a house on Quincy Place NW, just off Florida Avenue. Duncan’s body was found in a second-floor bedroom; Picott escaped but died at a hospital on March 5.

“It looks like those two homicides, in all likelihood, they will close,” Newsham said.

The chief said the earlier media reports noting Sweatt’s apparent confession should have prompted quicker action by authorities in the Quincy Place fire investigation. “He had said something to a reporter, saying he was involved,” Newsham said. “It was years ago that information had become available, and I would suggest that, at that point, probably some additional follow-up should have been done — and it was not.”

Prosecutors in the District declined to comment on the case. The U.S. attorney’s office in Maryland, which prosecuted Sweatt in the 2005 case, did not respond to interview requests. Sweatt’s attorney with the Federal Public Defender’s office in Maryland also did not respond to inquiries.

Picott’s son, Rodney Picott, a police officer in New York, said he was surprised by the new finding of homicide. He was 18 when the fire broke out and he managed to escape the burning house along with his sister, stepsister and stepbrother. His sister and stepsister, both 19 at the time, suffered serious burns.

Picott said he had questioned the D.C. fire investigation from the beginning and noted that his father had stopped smoking years before the fire. He said one investigator estimated the time of the fire at 2:40 a.m., but Picott recalls being up at that time and watching a movie. He put the fire closer to 4 a.m., the time listed by police in Monday’s statement.

Picott, who works for the Clarkstown Police Department, about 35 miles north of New York City, said fire investigators told him that Sweatt, while negotiating a plea, told prosecutors that he set the Quincy Place fire, but the case was not included in the final plea deal. He also said the investigator told him that the suspect had shown up at the funeral-home viewing for his parents, but did not go inside.

Even without a public confession, Picott said “it was comforting to know he would spend the rest of his days in a prison cell.” But he said it was “heart-wrenching” that for so long the cause of the fire was listed as an accident. “There is no chance my father woke up at two in the morning and had a cigarette,” he said.

The arson fires attributed to Sweatt didn’t appear on the police radar until the early 2000s, and they put the region on edge. Police arrested Sweatt in 2005 in connection with fires in the District, Maryland and Virginia. The former fast-food restaurant manager would light plastic jugs with gasoline and leave them on front porches. It often took up to a half-hour before the fire bombs exploded. Two people were killed in those fires.

Investigators collected clues at several fires in the region, including a device used to set fires, a single strand of hair and pants and a hat Sweatt had worn at one of the scenes. Police linked DNA to Sweatt and the evidence.

At his sentencing in September 2005, Sweatt apologized but did not offer a motive. His attorney at the time said he was “laboring under a psychological compulsion.”

In the City Paper article, published in 2007 and titled “Letters from an arsonist,” the man reportedly detailed how he set Quincy Place afire and also described other fires.

“There were different reasons for most of the fires,” one letter to the City Paper reads. “It could be because of one feeling the need to have power about something or someone. . . . I might like a particular style of a house and wish one day to own it (but it’s only a dream). Fire is a tool to destroy and some house fires also becomes my fantasy of people scrambling to exit windows and sort-of feel like they need my help so I stay and watch.”