Friends and neighbors Saturday remembered a woman killed in an apartment fire in the South End as caring and selfless — the kind of person who would greet them with a smile and be quick to hold the door open to let them into the building.

The woman, whom fire officials described as middle-aged, was found dead in the bathroom of a third-floor apartment at 109 East Canton St., after a fire broke out in the room, according to the Boston Fire Department. A cause of death has not yet been established for the woman, who was not identified, and what started the blaze remained under investigation, a department spokesman said.

Firefighters arrived at about 6:15 a.m. to the five-story, 20-unit brick apartment building to put out the blaze, according to Steve MacDonald, spokesman for the Boston Fire Department. The apartment had a working smoke alarm system, which helped alert officials to the one-alarm fire, he said, and firefighters extinguished the blaze within 20 minutes of their arrival.

The fire was confined to the third-floor apartment with damage estimated at $100,000, he said.

A man and child who were also in the apartment at the time of the fire were taken to a nearby hospital for evaluation, but did not have life-threatening injuries, said MacDonald. No other injuries were reported.

Officials said they could not confirm the relationship between the man and the victim. But neighbors and friends said the child, a young girl, was the daughter of the woman who was killed.

“She was a lovely lady, very respectful, and loved her daughter deeply,’’ said Mirlene Chery, who had lived in the building for nine years before recently moving out. “I feel sorry for the family and everyone else that’s suffering because of her loss.’’

Standing outside the scene of the fire Saturday afternoon, Chery said when she lived in the building, she would see the woman regularly with her daughter.

“It’s unfortunate another life was lost,’’ she said, visibly upset.

Angelisa Andrades, who lives on the building’s fourth floor, said she opened her door after hearing the alarms going off early in the morning. She quickly slammed her door shut when a thick, acrid smoke poured into her apartment, she said.

“It was super black,’’ she said. “The smoke was like super harsh. It was the smoke that it would get into your throat.’’

Andrades got her two children out of the building and into her car parked out front, and then ran back into the building to help other residents get out.

She said the man who was in the apartment when the fire broke out was trying to get back inside the smoke-filled apartment to rescue the woman. When Andrades later saw the man outside, she said he appeared burned and looked “out of it.’’ She said he kept saying “she’s inside, she’s inside. Help me, she’s inside.’’

Andrades said she did not know the woman’s name but described her as very nice. She said she had run into her the night before the fire.

“She was super friendly. Super, super nice. She was just so charming,’’ Andrades said. “You could just feel her presence when she walks into the room. It’s sad. It’s really sad.’’

The death marks Boston’s third fatal fire so far this year and the 20th statewide, MacDonald said.

Following a recent spike in fatal fires in homes lacking working smoke alarms, state Fire Marshal Peter J. Ostroskey made a public plea on Friday urging residents to check their devices.

Adrian Garcia, 18, who lives on the first floor of the building, said the victim would often hold the door for him and let him into the building with a smile.

“She was just a nice person,’’ he said. “I liked her.’’

Paulette Sadberry, who lives in the building next door, said she knew the woman from seeing her in the neighborhood.

Standing below the shattered windows of the apartment, she remembered the woman for her kind demeanor.

“She was a good mother and she was a hard worker,’’ she said. “If she saw you she would talk to you. You know, a smiley face . . . She was a very nice person, I just can’t understand this.’’

Steve Annear can be reached at steve.annear@globe.com. Nicole Fleming can be reached at nicole.fleming@globe.com.