Six people were killed when two World War II-era planes collided in mid-air at a show in Texas and fell to the ground in a fireball, authorities said on Sunday.

“According to our Dallas County Medical Examiner, there were a total of 6 deaths in yesterday’s incident at the Wings over Dallas air show,” Dallas County Judge Clay Jenkins said on Twitter Sunday.

The Dallas Executive Airport incident involved a Boeing B-17 Flying Fortress and a smaller Bell P-63 Kingcobra, the Federal Aviation Administration said Saturday.

Dallas Mayor Eric Johnson called the incident “a terrible tragedy,” adding that videos of the incident were “heartbreaking.”

Several videos posted on social media showed dramatic scenes of the smaller aircraft flying towards and crashing into the lower-flying B-17 as both flew in loops around the airport.

After the collision, the planes appeared to break up into several large pieces before falling to the ground and exploding in a fireball that created a huge cloud of black smoke.

The crash scattered debris across the airport grounds, as well as onto a mall and a nearby freeway that was closed for hours, Johnson said.

Fire and rescue vehicles were already at the fair in an emergency and responded immediately, the authorities said.

An estimated 5,000 people attended Saturday’s event, which was held a few miles south of downtown Dallas. Air show events scheduled for Sunday have been cancelled.

The FAA said its agents and the National Transportation Safety Board were investigating the incident.

Hank Coates, the chief executive officer of the Commemorative Air Force (CAF), whose aircraft were involved in the accident, said the B-17 “usually has a crew of four or five,” while the P-63 is piloted by a single pilot.

He said the pilots who operate the planes during such shows are experienced volunteers with “very thorough training” and are often retired military pilots.

The Allied Pilots Association, American Airlines’ collective bargaining officer, previously confirmed that two of its retired members died in the accident.

Both planes were based in Houston, the Dallas Morning News reported.

The CAF has about 180 vintage aircraft, according to Coates, which fly about 6,500 hours a year at air shows, veteran honors and training flights.

The B-17, a four-engined bomber, played an important role in defeating Germany in the air during World War II. With a reputation as a workhorse, it became one of the most-produced bombers of all time.

The CAF said the B-17 involved in the crash, which sported a Texas Raiders logo on its nose, was one of only five of the bombers still able to fly out of the 12,731 bombers originally built.

The P-63 Kingcobra was a fighter aircraft developed by Bell Aircraft in the same war but used in combat only by the Soviet Air Force.

Safety has long been an issue at air shows.

Seven people died on October 2, 2019 when a B-17 crashed during a classic car show in Connecticut.