(CNN) — Two World War II-era military planes collided in midair and crashed at Dallas Executive Airport during an airshow Saturday afternoon, killing all on board.

More than 40 fire rescue units responded to the scene after the two vintage planes — a Boeing B-17 Flying Fortress and a Bell P-63 Kingcobra — went down during the Wings Over Dallas airshow.

In video footage of the crash that was described by Dallas’ mayor as “heartbreaking,” the planes are seen breaking apart in midair after the collision, then hitting the ground within seconds, before bursting into flames.

Here are the latest developments as investigators from the National Transportation Safety Board are due to arrive at the scene Sunday.

What we know about the crew members killed

The Federal Aviation Administration said the crash took place at around 1:20 p.m. Saturday. The number of casualties has yet to be confirmed, according to Dallas Mayor Eric Johnson on Saturday.

The Allied Pilots Association — the labor union representing American Airlines pilots — has identified two pilot retirees and former union members among those killed in the collision.

Former members Terry Barker and Len Root were crew on the B-17 Flying Fortress during the airshow, the APA said on social media.

“Our hearts go out to their families, friends, and colleagues past and present,” the union said. The APA is offering professional counseling services at their headquarters in Fort Worth following the incident.

In a Saturday news conference, Hank Coates, president and CEO of the Commemorative Air Force, an organization which preserves and maintains vintage military aircraft, told reporters that the B-17 “normally has a crew of four to five. That was what was on the aircraft,” while the P-63 is a “single-piloted fighter type aircraft.”

“I can tell you that it was normally crewed,” Coates said. “I cannot release the number of people in the manifest or the names on the manifest until I’m released to do so by the NTSB.”

The Commemorative Air Force identified both aircraft as based in Houston.

“Currently we do not have information on the status of the flight crews as emergency responders are working the accident,” a statement from the group said, adding it is working with local authorities and the FAA.

No spectators or others on the ground were reported injured, although the debris field from the collision includes the Dallas Executive Airport grounds, Highway 67 and a nearby strip mall.

Rare aircraft involved

The B-17 was part of the collection of the Commemorative Air Force, nicknamed “Texas Raiders,” and had been kept in a hanger in Conroe, Texas, near Houston.

It was one of about 45 complete surviving examples of the model, only nine of which were airworthy.

The P-63 was even rarer. Some 14 examples are known to survive, four of which in the US were airworthy, including one owned by the Commemorative Air Force.

More than 12,000 B-17s were produced by Boeing, Douglas Aircraft and Lockheed between 1936 and 1945, with nearly 5,000 lost during the war, and most of the rest scrapped by the early 1960s. About 3,300 P-63’s were produced by Bell Aircraft between 1943 and 1945, and were principally used by the Soviet Air Force in World War II.

NTSB launches go-team

The FAA was leading the investigation into the air show crash on Saturday, but it was set to be turned over to the NTSB once its team reached the scene, Coates said.

On Saturday evening, the NTSB said it is dispatching a team to investigate the collision. The team, which consists of technical experts who are regularly sent to plane crash sites, is expected to arrive on Sunday, the agency said.

According to Coates, the individuals flying the aircraft in CAF airshows are volunteers and follow a strict training process. Many of them are airline pilots, retired airline pilots or retired military pilots.

“The maneuvers that they (the aircraft) were going through were not dynamic at all,” Coates noted. “It was what we call ‘Bombers on Parade.”

“This is not about the aircraft. It’s just not,” Coates said. “I can tell you the aircraft are great aircraft, they’re safe. They’re very well-maintained. The pilots are very well-trained. So it’s difficult for me to talk about it, because I know all these people, these are family, and they’re good friends.”

Mayor Johnson said in a tweet after the crash, “As many of you have now seen, we have had a terrible tragedy in our city today during an airshow. Many details remain unknown or unconfirmed at this time.”

“The videos are heartbreaking. Please, say a prayer for the souls who took to the sky to entertain and educate our families today,” Johnson said in a separate tweet.

The Wings Over Dallas event, which was scheduled to run through Sunday, has been canceled, according to the organizer’s website.