Correction: A previous version of this story misidentified the name of the apartment complex where the defendant set 25 fires. It is Southern Towers, not Seminary Towers.

The fires went on for months.

Always at night, always in hallways or trash rooms. Twenty-five times, residents of Alexandria’s Southern Towers apartment building staggered down the stairs after yet another blaze.

Then, on Dec. 6, 2013, a large freezer box was set on fire outside a trash room on the seventh floor, causing a larger blast than usual. Smoke filled the hallways of the 450-apartment, 1,500-resident building. The Bureau of Alcohol, Tobacco, Firearms and Explosives was brought in. Agents working out of an empty apartment spent so much time in the building that residents thought they lived there.

Cameras were installed, and one caught Shirley Vigneau, 74, entering a trash room one Tuesday night in March with a small plastic bottle and a lighter in her hand. When she left a minute later, fire was coming from the room.

The December fire “was very frightening,” she told investigators when they came to her apartment. The perpetrator is “probably a man,” she said, as most arsonists are.

This one wasn’t. Confronted with the evidence of her involvement, Vigneau confessed.

Vigneau pleaded guilty to one count of arson on Thursday and was given a suspended sentence and indefinite probation, along with an order to pay $9,873.16 in restitution. In court, shaking and frail, she said only that she was sorry and that she agonized daily over what she did.

“This is an extraordinary case, both in terms of the case and the defendant,” prosecutor Molly Sullivan said in court. Southern Towers, packed with people and devoid of sprinklers, has lost tenants because of the “fear, dread, and uncertainty” of the fires, she said.

Indeed, the circumstances behind the fires, which did not cause any deaths, are complex. Vigneau never stayed to watch, she told authorities; she also said that she wasn’t proud of the fires and that she never meant to hurt anyone.

“I think I was angry,” she told investigators, according to court testimony. “It was loneliness, not having any family.” Sometimes she would see fliers for community events and become frustrated by her isolation, she said. “It was a way of fighting back, maybe, saying I count.”

For most of her adult life, Vigneau worked as a legal assistant at the law firm Wilmer Hale, according to court documents. In one of several letters to the court sent by former colleagues, an employee described her as “ladylike, impeccably groomed and articulate, intelligent, law-abiding, and responsible.” All the letters expressed shock that Vigneau could have committed a crime.

But Vigneau retired from the firm eight years ago, her attorneys said. She had lived alone in the same Southern Towers apartment for nearly four decades. An only child from Lynchburg, Va., with no extended family, she married and moved to Alexandria in 1967.

Ten years later, divorced, she landed in Apartment 717. Her mother died in 1996; her father in 1975. Her ex-husband, with whom she had a friendly relationship, died in 2003. She’d never remarried; she had no children. She saw a psychiatrist but came away only with a prescription drug problem, prosecutors said in court.

Vigneau left jail for an extended-stay hotel while she looks for a new place to live — a task made difficult, her attorney noted, by her new record.

She has, however, gained something of a forced support system. As part of her probation, she will work with a geriatric health team through Alexandria’s community services board.

“You have kept everything inside you, and as a result it’s eaten away inside you,” Judge John E. Kloch told her at Thursday’s hearing.

Most of the defendants he sees are young people in danger of throwing their lives away, Kloch said. Pointing to the supportive letters from her former co-workers, he said, she was different. She must reconnect to the life she had. “Let them try to help you,” he said. “It would be a tragedy for you to end up where you are again just because you don’t want to talk.”

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