Family and friends were in mourning Sunday as the identities of some of the five people killed in Maine’s deadliest fire in a generation were confirmed and authorities continued to work on identifying a cause.

Investigators with the State Fire Marshal’s Office and Portland Fire Department resumed work at the burned husk of the two-family apartment house at 20-24 Noyes St., trying to narrow down the area where the fire originated Saturday morning and determine whether the cause was accidental or intentional. A dog trained to sniff for accelerants such as gasoline was walked through the building Saturday night and again Sunday, but officials would not say whether the dog identified any.

Agents with the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives will bring their expertise to bear on the investigation Monday. The federal agency was brought in because of the severity of the fire, said Stephen McCausland, spokesman for the state Department of Public Safety.

Authorities also walked through the building Sunday looking for evidence of smoke and carbon monoxide detectors to assess whether the building met safety codes. The city would not release inspection records for the building at the request of the State Fire Marshal’s Office because they are a component of what could become a criminal investigation, a city spokeswoman said.

The fire broke out shortly after 7 a.m. Saturday in the apartment house, which neighbors have complained hasn’t been maintained well for years. There were six tenants at 20 Noyes St., authorities said.

People who knew the victims reacted Sunday with shock, sadness and a desire to know what went wrong.

“What’s the cause?” an emotional David Bragdon Sr. of Rockland asked Sunday afternoon, in front of the building where his son David Bragdon Jr. died. And he had other questions, according to WCSH-TV. “Were there smoke detectors working? … Did the landlord do what he needed to do to make sure something like this never happened?”

The deaths also left one of the survivors feeling a sense of guilt that he could not have done more.

“The smoke was coming so fast … so intense. … The fact I didn’t have one minute to kick in doors and save you will eat at me for the rest of my life,” Nathan Long said in a Facebook post, referring to David Bragdon Jr., Ashley Thomas and Nikki Finlay, who lived in the building with him and died in the blaze.

Long awoke to smoke Saturday morning and, as flames started to fill the building, made it out a back window and jumped to safety from a porch roof, as did another tenant with a second-floor bedroom, Kyle Bozeman.

David Bragdon Sr. said Sunday that officials told them his 27-year-old son was among the victims. The younger Bragdon had lived at 20 Noyes St. since this spring and worked at the Great Lost Bear, a restaurant and bar on Forest Avenue.

Kevin Bragdon, 20, on Saturday described his brother as a “kind-hearted person. He would give you the shirt right off his back.”

He and his family became emotional when they talked about how David Bragdon Jr. had taught himself how to play guitar on an instrument he had gotten as a high school graduation present.

Thomas was a wedding photographer who studied at the prestigious Hallmark Institute of Photography in Massachusetts and ran a business with her close friend Mat Garber, called Mat & Ash Photography.

“She’d make your head turn just hearing her laugh, or watching her walk into a room and light it up with smiles,” said Garber, who was in the Midwest when the fire occurred and on Sunday was headed back to Portland.

Jo Moser, owner of Greenlight Studio on Dartmouth Street, where Finlay had worked for the past six weeks, said Finlay had a lightness and joy about her.

“She was just beautiful, very positive and competent,” Moser said.

McCausland said Sunday evening that the man severely burned in Saturday’s fire, Steven Summers, 29, of Rockland, was in critical condition Sunday night at Massachusetts General Hospital in Boston. Summers was visiting friends at the house Friday night. He escaped by jumping through a second-floor window.

The sixth resident of the apartment, Justin Irish, wasn’t home at the time of the fire. The other two people who died may have been friends of a tenant who stayed overnight, Long said.

Authorities said everyone believed to have been in the building at the time of the fire has been accounted for.

Snow fell Sunday on the charred remains of the apartment house as fire investigators resumed their probe into the cause and the state medical examiner worked to confirm the victims’ identities and causes of death.

The fatalities were the most to have occurred in a Maine house fire since 1984, when one adult and five children were killed in a fire in Hartland. A mother and six children died in Eagle Lake in 1974.

In 1963, six children were killed in Portland’s deadliest fire on record.

The State Fire Marshal’s Office said it was too early to say whether Saturday’s fire was suspicious. State Fire Marshal Joe Thomas would not say whether the dog taken through the building detected any accelerants.

“First and foremost is to find the area of origin and then the point of origin and then determine what it was that could ignite the fire,” Thomas said. That includes electrical wiring, heating systems and other sources.

“In order to get into whether or not it’s arson you have to be able to eliminate all those accidental causes. You have to determine that nothing here is likely to cause a fire,” he said.

The fire burned hot and fast, but Thomas said that says less about what started the fire than about the amount of combustibles inside.

“Throw a little breeze and wind in and it intensifies things as well,” he said.

Long, one of the survivors and the tenant who had lived there longest, said he had no idea what might have started the fire. Contrary to reports, he said the current tenants had kept the building cleaner than others did in previous years.

Sunday’s snowstorm didn’t help the investigation.

“We’re looking for things that are already destroyed, and you put water on top of it and it changes the landscape of what you’re working with,” Thomas said. But the accelerant-sniffing dogs are able to pick up a scent even if the scene is wet, he said.

Thomas said the side of the building where the porch is located is most heavily damaged, and that is where investigators will start looking for the origin of the fire. Also, eyewitnesses told investigators the fire appeared to have started on the front porch.

Two teams entered the building Saturday night, one from each side, to search for evidence of smoke and carbon monoxide detectors, Thomas said.

“Someone made mention of … not hearing anything when they were made to realize there was fire in the building,” Thomas said. But he noted that at 5 p.m. Saturday, he could hear the telltale beeping of a detector coming from somewhere.

Portland officials noted that the fire occurred the day before the end of daylight saving time, which is when fire safety advocates encourage people to change the batteries in their smoke detectors.

Portland Fire Chief Jerry LaMoria said the investigation was in a preliminary stage and could take several days before authorities know how the fire started.

The length of the investigation could depend on what kind of tests need to be done, such as on electrical equipment, and whether investigators will try to re-create fire conditions.

The 94-year-old house is in an area where many University of Southern Maine students live.

Officials have not released the names of the people who died but said none of them were USM students.

On Sunday, a mattress could be seen jammed against a wall on the first floor; a broken computer terminal sat on the ground outside the home. On the front porch was a burned gas grill and detached propane tank, and a scorched bicycle frame.

City spokeswoman Jessica Grondin said officials were looking through records at City Hall to determine whether there is a history of safety or code violations at the house. Those records are not available online, and Grondin said they are part of what could become a criminal investigation. She said the State Fire Marshal’s Office would not allow her to release them.

The building is owned by Gregory Nisbet, who would not take any questions during a brief telephone interview Saturday night and could not be reached Sunday.