Two Malaysian military helicopters collided midair and crashed during a training session on Tuesday, killing all 10 people on board and injuring a swimmer in a pool, authorities said.

The helicopters were rehearsing at a naval base in northern Perak state for the navy’s 90th anniversary celebration when the accident occurred, the navy said in a brief statement. “All victims were confirmed dead on site,” it said.

A video circulating on social media purported to be of the incident shows several helicopters flying low in a formation. One of the helicopters veers sideways and clips the rotor of another helicopter, causing both to plunge and crash. A local police officer, who declined to be named because he isn’t authorized to speak to the media, confirmed the footage is genuine.

The two choppers were mangled beyond recognition. Rescue workers sifted through the debris to find the bodies. The victims included three women and seven men aged between 26 and 41, authorities said.

Prime Minister Anwar Ibrahim said the nation mourned over the “heart and soul-wrenching tragedy.” He said the navy will immediately carry out an investigation to find the cause of the incident.

Seven of the dead crew members were aboard an AW139 maritime operations helicopter, the navy said. That aircraft is produced by AgustaWestland, a subsidiary of the Italian defense contractor Leonardo. The other three were on a Fennec lightweight helicopter, manufactured by European multinational defense conglomerate Airbus.

Defense Minister Mohamed Khaled Nordin said that the anniversary celebration this Saturday will be cancelled and replaced with Friday prayers for the victims.

Khaled said the two helicopters were among seven rehearsing when the collision occurred. One crashed into an empty sports field, while another plunged into a nearby swimming pool. A swimmer sustained injuries after he was hit by debris from the plane, he added.