BANGOR , Maine — Investigators are working to pinpoint the cause of a fire Thursday afternoon that led to heavy damage on the first floor of an apartment building on Sidney Street and smoke and water damage throughout the rest of the structure.

The fire at 90 Sidney St. was reported shortly after 4 p.m. by a passerby, who saw smoke pouring from some of the windows, Assistant Fire Chief Darrell Cyr said.

Cyr said that the fire was knocked down within less than five minutes, partly because Sidney Street is so close to the city’s Central Fire Department.

“We were very fortunate,” he said. “It didn’t take us long to get there.”

The fire remained under investigation Thursday night, Cyr said. He said Sgt. Scott Richardson of the state fire marshal’s office visited Thursday but wanted to interview the tenants before closing the investigation.

The renters were not home when the fire broke out and had yet to be found as of 8 p.m. Thursday. Friends who stopped by the apartment building after the fire are helping look for them, Cyr said.

The apartment on the second level was vacant, Cyr said.

While no humans were injured in the fire, a dog and all but one of the three or four puppies she recently gave birth to died, Cyr said.

Firefighters were able to save one of the puppies by administering oxygen, he said. The surviving puppy was taken by Bangor police to a nearby veterinarian’s office.