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Fourteen people were injured – six of them critically -- in a building explosion in Minneapolis that sent flames shooting 20 feet into the air Wednesday morning, authorities said.

The ferocity of the blaze pushed back firefighters, who had to battle the flames from the outside with hoses. Not everyone was accounted for, officials said.

The three-story building on Cedar Ave. South housed a mosque, a halal market and 10 apartments. Rescuers rushed there after a blast and fire were reported at 8:16 a.m.

"When crews arrived, there were victims already outside the structure. There was heavy fire on the second and third floors," Fire Chief John Fuertel said.

Firefighters got into the first floor, but "the conditions became very untenable very quickly," Fuertel said. "They did the best they could."

Firefighters also coped with sub-zero temperatures and wind chills near minus-20 degrees.

Some residents were rescued with ladders. Others jumped or fell. It's unclear how many people might have been trapped inside and a family center was being set up to try to determine if anyone was still missing.

The cause was under investigation.

"We don't know the reason for the fire," said Minneapolis Mayor R.T. Rybak. "This is obviously a very, very difficult and potentially tragic event."

First responders at the scene of a building explosion and fire in Minneapolis on Jan. 1, 2014.Jim Leinfelder / NBC News