An early morning house fire in western South Dakota Saturday killed five children who may have been attending a sleepover, according to reports.

The victims, ages 6 to 9, died after the fire broke out around 4:30 a.m. in a multi-family home in Spearfish, near the Wyoming border.

“What we’ve seen this morning is an unspeakable horror,” Spearfish Police Lt. Boyd Dean said, according to KELO-TV.

The station posted a video in which fire officials said at a news briefing that they heard there was a sleepover at the house, but weren’t sure.

Authorities said adults were home at the time. They tried to rescue the children and then were taken to the hospital.

Firefighters weren’t able to save the children because of the volume of fire, authorities said.

Some of the victims were found on the second floor of the two-story house.

It took firefighters over an hour to control the fire, authorities said.

The house was fully engulfed in flames when firefighters arrived, KNBN-TV reported.

The cause of the fire was not known, the station reported.