They came in the middle of the night, arriving as the fire was being extinguished at a Plymouth house two weeks ago.

Someone was already rescued from the flames and on his way to the hospital. The quiet scene would now be taken over by workers rarely known to the public.

Since 2005, 19 firefighters have comprised a team that investigates fires in Hennepin County. Their job is to determine cause and origin of the fire, which sometimes can lead to the opening of an arson case.

The head of this group is longtime Golden Valley firefighter David Gustafson, who has also been part of the team before taking the reins. Last month he was named Fire Investigator of the Year by the state’s chapter of the International Association of Arson Investigators.

“I want to thank my wife and kids because I’m always leaving for fires at a moment’s notice,” he said. “And if not for my team and the good work we do, I wouldn’t be winning this award.”

Gustafson, 50, who also works as an on-call firefighter and investigator for Robbinsdale, led his team on 36 fire calls at homes and businesses in 2016. The causes ran the gamut: five were arson, two from smoking, six were electrical or from unattended outside cooking and seven were from stoves.

The other 16 calls were either to assist at the scene or were undetermined. Many of the fires were in Plymouth or Brooklyn Center, said Gustafson.

The investigators work with law enforcement if necessary. The team is mostly firefighters, but there is also a police officer and attorney on board, said Gustafson.

Investigating a fire is similar to processing a crime scene. The area is systematically surveyed, photos taken, wiring mapped, wreckage rummaged through for any clues. Firefighters receive education on things to look for during the fire that might help investigators.

Police are often asked to interview witnesses “because they are good at that,” said Gustafson. They may also find a person who shot the fire with their cellphone.

“Lots of things can point to the fire’s origin,” said Rick Hammerschmidt, the Golden Valley deputy fire chief who nominated Gustafson for the award. “It can be daunting. Sometimes buildings are burned so bad you don’t have a lot to work with.”

Firefighters who want to become investigators don’t need prior experience because they will attend several training sessions. The goal is for each firefighter to earn certification through the State Fire Marshal’s Office and the state Board of Fire Training and Education. Some even have national and international certification.

“They learn more from every fire,” said Hammerschmidt. “They learn from others with different training.”

Only one other metro county has a team similar to Hennepin County. It formed in response to a need to help cities without investigative resources. Before the team was created, the State Fire Marshal’s Office would most likely be responsible for examining fires.

Gustafson admitted it’s a bit of a struggle to get firefighters to volunteer for the team. It takes time and commitment, and “I need people who will respond in the middle of the night.”

“They are paid on-call,” he said. “But they are working their real jobs during the day.”

Firefighting itself is more challenging these days, said Hammerschmidt. Homes have more plastic and petroleum products that burn faster and hotter. The lightweight construction of homes makes them collapse quicker, he said.

The hardest part of the job is coming across a fatality, which Gustafson did in 2015 when a child was killed in Brooklyn Center.

His dedication through any investigation made nominating Gustafson for the state award a no-brainer, said Hammerschmidt.

“He’s a hard-working guy,” he said. “Somebody you can count on.”