A space heater had been running for days between the living room and dining room in the north Minneapolis duplex where five young children died, the same area where the fire appears to have started.

A Fire Department incident report obtained by the Star Tribune Monday said the intensity patterns of the fire indicate it started in that area but the exact source of the fire was difficult to determine because so much of the apartment was destroyed. The Feb. 14 blaze was so heavy that it burned a 6-foot circle through the third floor, where several children were sleeping, and destroyed most of the stairs leading there.

Investigators said in the report that smoke detectors were present in the unit, but it was unclear if they worked during the fire.

Troy Lewis, the only adult in the apartment, was sleeping just before 5 a.m. when smoke and possibly the sound of children calling out awakened him, according to the report. Opening the door to his bedroom, he encountered plumes of smoke and enough fire to make the unit impassable. He then jumped out of a window and ran up the rear stairs to try to rescue his seven children.

Lewis grabbed two children at the doorway to the third-floor rooms but was overcome by smoke before he could rescue others.

Less than five minutes after the first 911 call was made, firefighters arrived on the scene. They located three children who were trapped on the third floor and two on the second floor. Three were already dead, and another two died at North Memorial Medical Center.

Surveying the scene after the fire, investigators found furniture reduced to springs below the third-floor bedroom, which was missing most of its floor. A second-floor bedroom door was completely burned through, with extensive charring covering the bedroom walls closest to the living room and dining room where the space heater was located.

"Ignition sources in the area are difficult to determine due to the heavy destruction of all contents," Fire Department investigator Bonnie Bleskachek wrote in the investigatory report. "The adult occupant did state that there had been a space heater in the archway beneath the living room and dining room that was plugged in and had been running for several days." It was plugged into the north wall of the living room.

Fire officials said last week that the cause of the fire is officially now "undetermined," but that nothing indicates it was intentional.

Five of Lewis' children were killed: Gwendolyn, 18 months, Troy, 3, Fannie, 4, Mary, 6 and Christopher, 8. Two others, 9-year-old Shaca and 5-year-old Electra, remain in the hospital in critical and satisfactory condition, respectively. Troy Lewis was released over the weekend.

Eric Roper • 612-673-1732

Twitter: @StribRoper