Serial arsonist Thomas A. Sweatt offered an apology yesterday as he was sentenced to a life prison term for carrying out a deadly string of fires, but he provided no clue in court about why he spent nearly three years igniting homes where people were sleeping.

It was the first time that Sweatt spoke publicly of the pain he caused by setting 45 houses and apartments on fire throughout the Washington region -- blazes that killed two elderly women and caused millions of dollars in damage. His words rang hollow, according to family members who said they want a motive and explanation.

Having exchanged the orange jail jumpsuit he wore at an earlier court appearance for a dark blue suit, white shirt and dark tie, Sweatt stood and leaned toward a microphone at the federal courthouse in Greenbelt. He turned toward the side of the courtroom where more than a dozen of his victims and their relatives sat.

In a soft voice, Sweatt said: "I'm very sorry for all of the harm that I've caused you. To those who have lost loved ones, I share your hurt, and I share your pain every day."

To anyone who harbors hatred toward him, Sweatt said he would ask God "to replace that hate with understanding." He also apologized to about a dozen members of his own family, some of whom had traveled from Texas and North Carolina to attend the hearing.

Sweatt, a 50-year-old former fast-food restaurant manager from Southeast Washington, pleaded guilty to murder and arson charges in June, less than six weeks after his arrest. His plea agreement with prosecutors called for a life sentence with no chance of parole; the judge yesterday tacked on nearly 136 additional years.

"There's nothing he could say that would be acceptable," said Carolyn Jones, 54, a daughter of Lou Edna Jones, the 86-year-old woman who was killed by the fire Sweatt set at her Northeast Washington home in June 2003. Jones said she left the courtroom as Sweatt stood to speak.

Her mother, known and beloved throughout the neighborhood as "Mama Lou," had lived in the house for 50 years. Jones said she would have liked to see Sweatt receive "the same sentence he gave my mother, the same way he gave it to her."

The other fatal fire was among the first fires that Sweatt admitted setting: a February 2002 blaze that claimed the life of 89-year-old Annie Brown of Northeast Washington. Sweatt admitted to setting fire to 44 more apartments and houses, as well as several vehicles, covering a wide swath of suburban Maryland, the District and Northern Virginia.

Investigators have quoted Sweatt as saying that he was addicted to setting fires and that he chose targets at random. They said he typically placed a plastic jug filled with gasoline just outside the front door and lighted a cloth wick. Sweatt was a thrill-seeker, authorities said, and sometimes watched from his car as the blaze spread. He knew the residences were occupied, they said, and he kept track of media accounts of the massive hunt for a suspect.

Sweatt's attorney, Assistant Public Defender John C. Chamble, told the judge that Sweatt could not stop setting the fires because he was "laboring under a psychological compulsion." Chamble compared Sweatt to a literary character, the peaceful Dr. Jekyll and murderous Mr. Hyde.

"You had the good Mr. Sweatt and the sick Mr. Sweatt," Chamble said.

In sentencing Sweatt, U.S. District Judge Deborah K. Chasanow told him that any mental health issues "can't in any way justify the actions you took, actions which terrorized a community."

Chamble asked the judge to recommend that Sweatt be placed in a facility where he could receive psychological treatment. Chasanow said she would make the recommendation. Federal prosecutors did not contest Chamble's request.

Sweatt, a former manager at a KFC/Pizza Hut on Bladensburg Road in Northeast Washington, has been jailed since his arrest April 27. Investigators said they tied him to the crimes through DNA evidence recovered at four arsons.

Within hours of his arrest, authorities said, Sweatt confessed.

During a news conference, Maryland U.S. Attorney Rod J. Rosenstein praised the Serial Arson Task Force, which included members of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives and local police and fire department investigators from Maryland, the District and Virginia. He also cited the work of the Naval Criminal Investigative Service, which helped connect DNA from a single strand of hair left at a September 2003 fire scene to Sweatt.

If the task force had not arrested him, "I have no doubt Mr. Sweatt would have set fire to more houses, and more people would have died," Rosenstein said.

Sweatt, of the 500 block of Lebaum Street SE, lived near a convenience store where he may have purchased a few of the jugs used to set fires, authorities said. The KFC/Pizza Hut was near many of the fire scenes, and Sweatt worked a shift that would have allowed him to strike in early morning.

Investigators said yesterday that they were relieved by the quick guilty plea and lengthy sentence. "Everybody had to put everything they had into this," said Tom Daley, an ATF agent and one of the lead investigators, recalling the many twists and turns of the probe.

In an interview at an ATF office in Lanham, Daley, other federal agents and local police and fire officials said the arson probe was the most taxing and complicated serial case they ever worked.

The agents and officers on the task force, which had more than 100 members during its nearly two-year existence, conducted lengthy surveillance details and, day after day, cruised neighborhoods before dawn. Authorities offered a reward of as much as $100,000 for information. Yet they never caught the arsonist in the act.

"Arsons are hard to prove," said ATF agent Scott Fulkerson, another of the lead investigators. "There is limited evidence left behind. Almost everything is consumed in fires. This covered three states and multiple jurisdictions. There was a lot of media coverage and great stress to the community. There was pressure felt by the task force to catch this guy. There was an ongoing threat to the community."

Investigators declined to discuss much of what they learned from Sweatt during a lengthy interrogation on the day of his arrest and during six subsequent debriefing sessions. They said he discussed a range of issues, pointed out places where he struck and provided a wealth of information about the mind-set of a serial arsonist.

In court yesterday, Sweatt was far less forthcoming.

"It would be better if he had sat down and kept his mouth shut. He's not sorry," declared Darlene Lloyd, the oldest of Lou Edna Jones's four daughters.

Lloyd and her sister, along with two other relatives of Sweatt's victims, addressed the judge at the sentencing. Lloyd told the judge that she learned from authorities that both of her mother's feet had been burned by the time firefighters found her in her bedroom.

"She lost both of her feet trying to get out of that big room," Lloyd said.

U.S. Attorney Kenneth L. Wainstein talks with Darlene Lloyd, left, and Carolyn Jones, daughters of Lou Edna Jones, who was killed by Thomas A. Sweatt.