An arsonist set the fire at an apartment building last week that killed an elderly woman, police said Monday.

Police said they have several good leads but would not say if they have any suspects.

Also Monday, Kristin Roberts, a third-floor tenant of the building, said she heard an argument between a man and a woman about two hours before the fire was discovered.

Roberts said the argument sounded like it was on the second floor, where the fire started.

The fire broke out about 10:30 p.m. Thursday at 527-529 Washington Ave., a six-unit, three-floor apartment building.

The case was being treated as a homicide, said Police Lt. Richard Rizzo, who confirmed the argument was part of the police investigation.

“Anytime people argue and there’s a fire several hours later, we take it seriously, ” he said.

However, Rizzo said it was too early in the investigation to know the significance of the argument.

He said police know one of the people involved in the argument was a tenant, but he would not identify the person or say if it was a man or woman.

Police are trying to determine the identity of the second person involved in the argument, Rizzo said. He said he did not know what the argument was about.

Rizzo declined to comment on how the fire was set, saying the investigation could be hurt if that information was made public.

He ruled out the possibility that the arsonist had targeted the elderly woman who died in the fire. But he wouldn’t comment on whether another tenant was a target.

Firefighters had to use ladders to remove Maude Holden, 73, and her husband, Peter, 77, from the building. Maude Holden died at Maine Medical Center soon afterward. Peter Holden was discharged Monday afternoon from Brighton Medical Center.

Still hospitalized with burns at Maine Medical Center on Monday were the occupants of one second-floor apartment: Catheryn Richards, 33, and Gary Thibeault, 39. Both were in satisfactory condition. Their 7-year-old daughter, Felicity, was released from the hospital during the weekend.

Roberts said she was playing cards with friends and family when she thought she heard something break inside the building.

Roberts said she told everyone in her apartment to be quiet, opened her apartment door and heard a man and a woman arguing, probably from the second floor. She couldn’t tell who was arguing.

She said the argument didn’t sound like it involved violence, so she closed the door, went back to the game and thought nothing of it.

Two hours later, Roberts heard Felicity Richards screaming from the second floor. Roberts’ boyfriend, William LeClair, who was about to go to bed, smelled smoke and saw the porch of their third-floor apartment was on fire.

Thibeault, interviewed by telephone from the hospital, said he did not hear an argument that evening and was not involved in one with Catheryn Richards. He said he was asleep when Richards discovered the fire and woke him up.

Rizzo said police now believe the fire started in a public area of the second floor, either in a hallway or on the porch. Investigators determined it did not start in Richards’ apartment. Originally, they were scrutinizing two heavily charred areas in Richards’ living room and dining room.

Rizzo said a Friday night interview with Richards helped investigators determine the fire was arson, but he would not explain what led them to that conclusion.

Saturday, investigators found evidence on the building’s second floor that strengthened their belief the fire was arson. Rizzo would not say what was found.

Catheryn Richards declined a request for an interview. Other efforts to reach former tenants of the building were unsuccessful.

Richards is the aunt of Stacy Richards, who was the target of Portland’s deadliest fire in 30 years, the 1992 Munjoy Hill blaze set by Virgil Smith. Smith, convicted of arson and four counts of murder, was Stacy Richards’ former boyfriend and the father of her child. Both escaped the fire.