Two children were killed in a fire at a duplex in Mankato, Minnesota, early Monday, city officials said.

The fire broke out at the upstairs apartment at 233 1/2 Bradley Street around 2 a.m.

A man and a child made it out safely, but two more children died in the fire, Mankato Public Safety said. An injured woman was airlifted to Hennepin Healthcare in Minneapolis.

"This is a heartbreaking tragedy, especially at the start of the new school year," Director of Public Safety Jeremy Clifton said. "We are working to determine the cause and will support those affected in any way we can."   

The children have not been publicly identified.

Everyone in the lower apartment got out safely.

The fire's cause is being investigated, though the city said it was likely not intentional. The State Fire Marshal is aiding the investigation.