Officials announced on Sunday that six people were killed after two vintage military aircraft collided in midair on Saturday and burst into flames in front of thousands of spectators during a World War Two commemorative air show in Dallas.

The 4,000 spectators were unharmed, and the victims’ names remained hidden.

Judge Clay Jenkins, the county’s top elected official in Texas, tweeted, “Authorities will continue working today on the investigation & identification of the deceased.” “Pray for their families and everyone affected,” was said.

The Boeing B-17 Flying Fortress bomber and the Bell P-63 Kingcobra fighter were flying at the Wings Over Dallas Airshow at Dallas Executive Airport when they crashed, according to a statement from the Federal Aviation Administration (FAA).

The B-17, a massive four-engine bomber, typically has a crew of four to five people, according to Hank Coates, president and chief executive officer of the Commemorative Air Force, a group devoted to the preservation of World War Two combat aircraft.

Coates added that the P-63 is a single-pilot American fighter plane, but he would not specify how many passengers were on board at the time of the accident.

Leah Block, the spokesperson for the Commemorative Air Force, which is dedicated to preserving World War Two aircraft and hosts the air show, said told the Fort Worth Star-Telegram that they are not sure why the accident occurred. Officials from the National Transportation Safety Board (NTSB) started an investigation on Sunday.

Video clips posted on social media showed the two aircraft colliding and crashing on the ground, engulfed by a fireball and a cloud of black smoke as a scattered cloud of shrapnel from the craft fell to a patch of scorched earth.

Previous fatal air show disasters have happened from both pilot and mechanical failures.