Fruetel said firefighters encountered heavy smoke and fire on the second and third floors of the duplex as they searched for victims and survivors. It took firefighters about 2½ hours to put out the blaze.
He said investigators were still trying Friday to determine a cause, but it appeared the fire started on the second floor, in the center of the building. He said a space heater was found on that floor, but he didn't know if it was in use before the fire began.
He said the building was up to code and smoke detectors were hard-wired and appeared to be working. He said he didn't know if the smoke detectors went off before this fire.
Gov. Mark Dayton issued a statement offering condolences to the family and friends of the victims, and thanking firefighters and other first responders "whose heroism saved lives this morning."
Minneapolis Public Schools confirmed two of the children killed were students at Bethune Community School, in the first and second grades.
City records show an inspection report last summer called for repairs to smoke detectors in the unit, among other violations. City spokesman Matt Laible said all of the violations were taken care of and that the unit's rental license was renewed just a week ago.
The owner of the property, Paul Bertelson, said Lewis and his family moved in about six to eight months ago.
"There were no disturbances. They were good tenants," he said, adding that the upper unit included two bedrooms on the second floor, and three on the third floor.
Bertelson's company identifies distressed or foreclosed properties, refurbishes them and rents to low-income families. He said the 102-year-old duplex had smoke and carbon monoxide detectors. The upper unit is heated by permanent baseboard units.
One of the more than 40 firefighters who responded to the scene was injured. Fruetel said he suffered a muscle strain.
The Red Cross was assisting three adults and four children who were displaced by the fire.