A Manchester firefighter injured in a three-alarm blaze Saturday night that killed one person is in a Boston hospital recovering from second- and third-degree burns over 35-40% of his body, officials said Sunday.

Capt. Steve DesRuisseaux became engulfed in flames during a flashover while rescuing a man from the second floor of a six-unit multi-family building on Dutton Street, Manchester Fire Battalion Chief Dave Fleury said.

“The actions of the crews last night went above and beyond the call of duty, and Captain DesRuisseaux’s were nothing short of heroic,” Fleury said.

The fire broke out around 6:08 p.m. Saturday at 8-10 Dutton St., a six-unit building.

“Upon arrival, an apartment building was found to be heavily involved in fire,” officials said in a statement.

After the fire was extinguished, a victim was found dead on the second floor back porch, New Hampshire State Fire Marshal Sean Toomey said.

Identification of the victim and cause and manner of death are pending an autopsy scheduled for Monday.

Firefighters rescued six people, including a baby, officials said. Two girls, two men and a woman were rescued from the third floor and a man helped down from the second floor by ladder, officials said. But DesRuisseaux’s breathing device got caught on a ladder, trapping him after he became engulfed in flames.

Lt. Scott Brassard climbed the ladder to try to free DesRuisseaux. Firefighter Joshua Charpentier noticed heavy fire coming from the second floor and decided to knock the ladder away from the building to prevent further injury to Capt. DesRuisseaux.

Lt Brassard sprained his ankle as a result of the 10-15 foot fall, but continued to battle the blaze throughout the incident, seeking medical treatment after the fact.

Capt. DesRuisseaux was transported by AMR ambulance to Elliot Hospital, then to Massachusetts General Hospital by UMass Memorial Lifeflight, officials said. He suffered 2nd- and 3rd-degree burns to approximately 35-40% of his body, and was receiving treatment for his injuries late Sunday.

Another person, a civilian, was sent to a Boston hospital for respiratory injuries.

Among the six people rescued was a baby. Video shows a firefighter clutching a young baby tightly, climbing down a ladder to safety. The same firefighter then climbed back up the ladder to rescue more people trapped inside.

Manchester Mayor Joyce Craig said Sunday her thoughts are with everyone affected by the blaze, including the family and loved ones of those injured and the individual who died.

“I am so grateful for the heroism shown by the Manchester Fire Department, including Captain Steve DesRuisseaux and Lieutenant Scott Brassard who were injured,” said Craig.

“Because of Captain DesRuisseaux’s quick actions, along with his fellow firefighters, the lives of six individuals were saved. The men and women of the Manchester Fire Department run into danger without hesitation to keep others safe. Their courageous and valiant efforts know no bounds, and their dedication to the residents of Manchester is unwavering. I am thankful for everything they do for our community.”

A man who gave his name as Ken said he was having a beer at his home on nearby Concord Street when he saw flames and heard fire trucks outside. He headed down the block and saw the flames.

“I saw them carrying that baby down,” said Ken. “They brought it down carefully, it looked like they came from a window in the middle of the third floor. I’ve never seen nothing like that.”

Jennifer lives on Concord Street, up the road and around the corner from the fire scene.

“We saw the flames and smoke,” Jennifer said.

She rushed over because her 70-year-old friend lived in the building that burned. The friend is OK and staying with relatives, she said, but fire personnel have been unable to find the woman’s pet cat.

“They are hopeful she may have gotten out,” said Jennifer.

Eleven fire departments helped the Manchester Fire Department battle the blaze — Allenstown, Auburn, Bedford, Concord, Derry, Hudson, Hooksett, Litchfield, Londonderry, Merrimack and Nashua.

Investigators with arson dogs were at the scene Sunday. A cause has yet to be determined.

Paul Feely is the City Hall reporter for the New Hampshire Union Leader and Sunday News. Reach him at pfeely@unionleader.com