(CNN) -- An explosion and fire at an apartment building in Minneapolis left 13 people hospitalized Wednesday. Six are in critical condition, Minneapolis Fire Chief John Fruetel told CNN.
There may still be people inside the building. Fruetel said not everyone has been accounted for.
Most of the injuries are from burns and "trauma of people that came out through the windows," said Robert Ball, a spokesman for Hennepin County Emergency Medical Services. Officials don't know whether people jumped out, fell or were pushed out by the explosion.
The three-story building was destroyed, with the roof and the second and third floors collapsing, Fruetel said.
The first floor contained a business; above it were 10 residential units, nine of which "would have been occupied at the time," he said.
Authorities did not know immediately how many people live in those units.
Heavy flames were seen coming out of the second- and third-story windows.
Hours after the incident, firefighters were still working to contain the blaze. They tried to enter the building but were able to get in only a little way.
Freezing weather in the city presents challenges and dangers for firefighters, since water used to try to put out the flames can quickly turn to ice, Ball said.
A mosque is located near the building. It was not immediately known whether it suffered damage.
It will be "some time" before officials can determine the cause of the fire, Fruetel said.
CNN's Fredricka Whitfield and Kara Devlin contributed to this report.