LOWER PAXTON TOWNSHIP, Pa. (AP) — Three passengers died after a charter bus carrying up to 50 people collided with a vehicle on an interstate in Pennsylvania, state police said.

The crash happened during heavy rain about 11:50 p.m. Sunday on southbound Interstate 81 in Dauphin County’s Lower Paxton Township, near Harrisburg, Pennsylvania State Police said. The bus flipped on its side.

The route being taken by the bus and the cause of the crash weren’t immediately known.

“This is considered a mass casualty incident,” Trooper Megan Frazer said. “We do have eight ambulances that assisted with us, as well as lifeline (emergency response), just because of how many people were involved and the amount of injuries.”

It was raining heavily at the time of the crash, WHTM-TV reported Frazer said.

Multiple people were taken to Milton S. Hershey Medical Center with injuries, but police didn’t immediately release further information. A message seeking comment was left Monday at the hospital.

The American Red Cross of Greater Pennsylvania was providing food, drinks and cots at the Chambers Hill Fire Department for passengers being discharged from the hospital. When asked how many people were being assisted, the Red Cross said “the situation is still evolving” and they were not able to release a number.