(CNN) A Boeing B-17 Flying Fortress and a Bell P-63 Kingcobra collided and crashed at the Wings Over Dallas airshow around 1:20 p.m. on Saturday, according to the Federal Aviation Administration.
Authorities responded to the incident at Dallas Executive Airport, Jason Evans with Dallas Fire-Rescue told CNN on Saturday.
The number of casualties in the crash was still not confirmed later on Saturday afternoon, according to Dallas Mayor Eric Johnson.
However, the Allied Pilots Association, the labor union representing American Airlines pilots, identified two pilot retirees and former union members among those killed in the collision.
Former members Terry Barker and Len Root were among the crew members on the B-17 Flying Fortress during the Wings Over Dallas airshow, the APA said in a tweet. The APA is also offering professional counseling services at their headquarters in Fort Worth following the incident.
"Our hearts go out to their families, friends, and colleagues past and present," their tweet said.
There were more than 40 fire rescue units on scene after the collision, the agency's active incidents page shows.
In a Saturday afternoon news conference, Hank Coates, president and CEO of the Commemorative Air Force, told reporters 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 being out of 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.
The FAA is currently leading the investigation, which is set to be turned over to the NTSB at approximately 9 p.m. when the NTSB team arrives at the scene, Coates said.
On Saturday evening, the NTSB said it is launching a go-team to investigate the collision. The team is expected to arrive on Sunday, the NTSB said in a tweet.
"Member Michael Graham will serve as spokesperson on scene," the tweet added.
"The maneuvers that they [the aircraft] were going through were not dynamic at all," Coates noted. "It was what we call 'Bombers on Parade'."
Johnson tweeted later on Saturday 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.
The event, which was scheduled to run through Sunday, has been canceled, according to the organizer's website.
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.
Southbound and northbound lanes of the highway were shut down after the incident, the Dallas Police Department said.
"This is not about the aircraft. It's just not," Coates said during the news conference. "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."
According to Coates, the individuals flying the aircraft in CAF airshows are volunteers and have a strict process of training. Many of them are airline pilots, retired airline pilots, or retired military pilots, Coates said.
The B-17 was part of the collection of the Commemorative Air Force, nicknamed "Texas Raiders," and had been hangered 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 United States 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.