The Maine fire marshal’s office has ruled that a fatal fire in Portland’s Munjoy Hill neighborhood last month was accidental.

Nicole Niehoff, 45, and Jake Millet, 40, died in the Sept. 21 fire at the three-story, four-unit apartment building on St. Lawrence Street. Investigators determined the fire originated on the second-floor back deck, which led to the apartment where Niehoff and Millett were found, according to a news release from the Office of the State Fire Marshal on Thursday.

Authorities have concluded their investigation but were not able to determine the cause of the fire because of the extent of the damage, Maine Department of Public Safety spokesperson Shannon Moss said. Investigators “cannot rule out the careless disposal of smoking materials as a possible cause of the fire,” according to the news release.

Marcella Look, a first-grade teacher at Georgetown Central School in Georgetown, said Niehoff had just started her second year teaching pre-K and kindergarten students. Look said Niehoff, who she mentored, was a mother and a passionate and creative teacher. She said Niehoff enjoyed using the school’s outdoor classroom and was excited to do new lessons and projects with her students.

“She was just getting to the point where things were clicking and everything was going well,” Look said.

Look said Niehoff’s laughter often rippled through the hallways of the school.

“You always knew when she was having a good time,” Look said.

A Facebook post by the Georgetown Central School said Niehoff was “a light in our school community, touching the lives of many children and families.”

Abbey McGilvery, a North Waterboro resident who went to Yarmouth High School with Niehoff, said her friend was always kind and caring to her classmates. She said Niehoff graduated from the high school in 1998, and that she and Niehoff were both involved in the school’s theater program.

McGilvery said though Niehoff often worked backstage, she also had an impressive singing voice.

“She was one of those people that sometimes could blend into the crowd, but once you got her one on one, she was really talkative and really, really funny,” McGilvery said. “That’s what I remember most about her.”

Millett’s former co-worker Andrew Kalet said his friend’s morning shifts at Tiqa, a pan-Mediterranean restaurant in Portland, were marked by his music.

Kalet, who now lives in New Hampshire, said Millett, a cook at the restaurant, introduced him to several new bands and genres during their hours together in the kitchen at the Old Port restaurant, where they worked together for about a year.

Carol Mitchell, the co-owner of Tiqa, said she has known Millett since he was a child. She said he went to Longfellow Elementary and Lincoln Middle schools in Portland with her son and was “the sweetest, nicest kid.”

Mitchell said Millett was one of the restaurant’s longest-term employees, working there for more than six years. She said he was a generous friend and dedicated, loving father.

“He was just a guy that everybody liked having around,” Mitchell said. “He always had a smile.”

Kalet said that, even when work became stressful, Millet was “kind of a goofball and had a positive spin on everything.”

“He was definitely all about treating people right and seeing good in everyone,” Kalet said. “And just getting a good laugh about anything. … He never really took anything too seriously.”