This article was reported by Peter Schworm, Laura Crimaldi, and Evan Allen of the Globe staff and Globe correspondent Claire McNeill. It was written by Schworm.
LOWELL — As friends and relatives mourned the deaths of seven people killed in a devastating apartment fire, investigators combed through the charred remnants Friday to determine what ignited the blaze and focused on whether the building’s alarm system was working properly.
Amid conflicting reports about whether the fire alarm system functioned correctly during the predawn fire, the city’s mayor said Friday that alarms had been triggered as recently as last week, when someone inside the building set off fireworks in a hallway.
That smoke set off an automatic alarm that brought firefighters to the building, Mayor Rodney M. Elliott said.
“There was a fire alarm system in there,’’ Elliott said from the scene on Branch Street on Friday. “It was activated, and the Fire Department actually came to this location.’’
Investigators said they had made progress in their search of the gutted three-story building, but had yet to pinpoint what caused the fire, or where it began.
“They’re not sure when they’re going to find the origin of the fire,” Elliott said. “That’s the challenge.”
The three-alarm fire broke out just before 4 a.m. Thursday and within minutes had engulfed the entire building, residents say. As dozens fled, the roof of the building collapsed, trapping residents on the top floor.
The seven victims, including a couple and three of their children, lived in two apartments on the third floor.
Some residents say they heard alarms. But a third-floor resident who narrowly escaped the fire said neither the smoke alarm in her kitchen, nor one in the outside hallway, worked during the blaze, and investigators said they were taking a hard look at whether the alarm system sounded.
Elliott said he spoke with a person on the second floor who said alarms could barely be heard.
“He said it was very faint,” the mayor said, noting that many air conditioners were running at the time.
The building’s smoke and carbon monoxide detectors passed an inspection in March 2013, a city official said. The city has not received any complaints about the building since the current owner, Sanjay M. Patel, bought it several years ago.
“This is not a building that raised red flags,” said R. Eric Slagle, director of Lowell’s Development Services Department.
Investigators are also examining whether fireworks played a role in the fast-moving fire. Authorities and witnesses have reported that an explosion preceded the fire and that it sounded like fireworks detonating.
“They haven’t ruled it out yet,” Elliott said.
One of the victims, Torn Sak, had fireworks in his third-floor apartment left over from his daughter's birthday last week, his brother, Thearan Sak, said.
Authorities said investigators were conducting a forensic analysis of the scene and reviewing witness statements and building records. The US Bureau of Alcohol, Tobacco, Firearms, and Explosives sent a special agent trained in fire investigations to assist the inquiry, and state troopers used dogs to search for any signs of an accelerant.
“This is a very robust search,” said a law enforcement official briefed on the investigation who requested anonymity.
Authorities have not released the names of the victims, but relatives have identified them as Torn Sak; his long-time partner, Ellen Vuong; and three of their children, Anthony, 12, Ryan, 9, and Sayuri, 7.
Two other siblings escaped.
Also killed were Tina Christakos, 38, and an elderly roommate, a man in his 70s.
Bobby Christakos, Tina’s uncle, held back tears as he recalled his niece as a “happy-go-lucky” spirit.
“She had tons of energy,” he said. As a child, she would run so quickly she would be next to him one minute and then be “a mile down the road.”
He said he would miss her “more than anything in the world,” as would her daughter, Felicia Neov, 22.
“Tina, you’re with Mom and Dad now,” he said. “They’ll show you the light.”
Across the street from the charred building, mourners created a small memorial.
Walking through the neighborhood with his mother, 7-year-old Ethen Lam carried a picture he drew of his classmate Sayuri Sak, showing her as an angel.
“I will miss her,’’ the boy said. “She is in a good place now.’’
Ethen said he sometimes played with Sayuri during recess at Charles W. Morey Elementary School, where both were in the first grade.
“She’s nice,’’ he said. “She’s kind.’’
Anthony Sak was a nice, quiet child, said schoolmate Monetry Vilavong, 12.
“There’s a lot of people who miss him already,” he said. “It was really sad because he was young. He only lived 12 years.”
Jose Rosado, 43, who lives next door, said Torn Sak was a popular presence on Branch Street, part of a tightknit neighborhood with a strong Cambodian presence.
“He’s going to be missed by so many people,” he said.
Rithy En, 34, said Sak was a devoted father whose children followed him like ducklings.
“Wherever he goes, they go,” he said. “He’s very well respected, a good, honest person. . . . Like a big Pooh bear, that’s him.”
He said his daughter often played with the Sak children. It hurt him, he said, to think that they were gone.
“They’re in a better home now,” he said.
He described the man in his 70s as a good man who did his best to provide for those close to him. He called him uncle, out of respect.
“He tried his best to find work to support that house,” En said. “He is very generous guy.”
Thursday’s fire was the deadliest in Massachusetts since 1994, when a wind-whipped fire killed seven people, including five children, in North Attleborough on Christmas Eve.
The blaze early Thursday left more than 50 residents homeless, with no possessions.
Kim Ou, 40, who escaped from the third floor, returned to Branch Street wearing clothes she bought that day using money from the Red Cross.
She had only been living in the building a month and lost everything, she said.
She and two of her children, Soriya, 11, and Vesna, 10, were rescued by firefighters from a bedroom window.
After a neighbor woke her up, yelling “Fire!,” she tried to lead her children out the back door but ran into a wall of smoke. She broke a window and screamed for help.
Ou could not see anything and could barely breathe. Her children were yelling for help. But then firefighters were at the window.
“There was no way to get out, only the window,” she said.
Odum Chaloeurn, 54, who lived on the second floor, woke to a thump from another apartment, and when he opened his bedroom door, saw his kitchen filled with smoke and fire.
He woke his girlfriend and another friend, and the three of them escaped out the back door, flaming debris hitting him in the back as he fled.
Outside, he saw a child thrown from a second-floor window into the waiting arms of three firefighters. He looked to the third floor, where he saw Torn Sak banging on the window, which would not open. Sak banged for about two minutes, he said, and then disappeared.
“I look again, and I can’t see him,” he said. Then he heard fireworks going off.
Minh Ngo Van, whose two roommates died in the fire, said he felt haunted by what happened.
“I feel no good,’’ Van said in broken English when he visited an American Red Cross office set up for the survivors. “I can’t sleep good. I keep thinking about it.’’
Van lived on the third floor with several roommates, including Christakos and the man in his 70s. Van said he was cooking noodles when he smelled smoke. He looked outside and saw flames flowing out of the building.
Van said he helped two other residents, including an elderly woman who appeared ill, down the stairs to safety.
When he looked down the hall toward the Sak apartment, he saw flames everywhere, and the ceiling starting to collapse.
He remained in shock about what happened.
“It burned so quickly,” he said. “Five minutes — the building go.’’