The March 1 fire at a Berwick apartment building that killed Berwick fire Capt. Joel Barnes was sparked by improperly discarded smoking materials, the state fire marshal said Friday.

The determination comes after a month of investigation by the fire marshal’s office, which cooperated with investigators from the federal bureau of Alcohol, Tobacco and Firearms.

Barnes died after he became trapped in a third floor room. The cause of his death was listed as hyperthermia, a fatally elevated core body temperature, said Fire Marshal Joe Thomas.

Investigators were unable to conclusively determine who discarded the cigarette or smoking item that caused the blaze, and no charges are expected, Thomas said.

“We assume somebody was out there smoking at some point in time,” Thomas said. “It certainly is fitting of the circumstances showing what cause and origin was.”

The cause and origin determination comports with video footage taken by neighbors shortly after the fire began showing intense flames from the rear porch area of the top level of the three-floor, six-unit building.

Barnes’s father said his son died after throwing himself on top of another fire fighter, Mitchell Manfredi, to shield him from the heat and flames. Manfredi survived.

Barnes and Manfredi had to be pulled from the building. Three other firefighters who entered the building with them managed to escape on their own. None of the residents were injured. He said Barnes, Manfredi and three other firefighters entered the building “primarily to make sure everyone was out.”

Barnes said his son was among a group of five firefighters who went into the building to fight the flames, and only three of them found a way to escape. Barnes and the firefighter he protected had to be pulled from the building at 10 Bell St.

A somber crowd of 2,500 firefighters, relatives, friends and other community members gathered March 10 at the Cross Insurance Arena in Portland to honor Barnes, believed to be the first Maine firefighter to have died in a fire in nearly four decades.