A fire swept through a condominium building in central Missouri overnight, killing four young children who were there to celebrate a birthday party, authorities said Wednesday.

People began calling 911 to report the blaze shortly after 11 p.m., and firefighters were at the four-story building within minutes, Osage Beach Fire Chief Jeff Dorhauer told the St. Louis Post-Dispatch. The fire was raging on the top floor, where the children were, and flames were shooting through the roof.

Firefighters rescued two adults from a third-floor unit — they weren't injured — but they had to go through a window to reach the children in the upstairs unit because the stairwell had collapsed, Dorhauer said. The kids — two 2-year-olds, a 4-year-old and a 5-year-old — were unresponsive. They were later declared dead at a hospital.

Dorhauer told St. Louis television station KTVI that the children were all related and that he believed one of them lived in the home. Their names and genders weren't immediately released.

"When it is children, and when it's more than one, it becomes personal," he said. "The guys tend to see their family as they're doing this. And sometimes it's very hard to get over."

The children had apparently gathered at the home for a birthday party earlier Tuesday. The mother of the child who lived in the home was at work when the fire broke out, the chief said. The father managed to escape, then tried to re-enter but was forced back by the heat and flames, he said.

The fire is under investigation but there's nothing to suggest there was anything criminal about it, Dorhauer said.

Firefighters from eight departments helped fight the blaze in Osage Beach, which is 135 miles southwest of St. Louis and one of several communities in the Lake of the Ozarks region. One firefighter suffered minor injuries when he fell off a ladder.