While the cause of a deadly collision between two vintage military aircraft at a Dallas air show to commemorate Veterans Day remains unknown, experts said Monday that the accident will likely renew discussion over whether additional safety rules are needed for such events.

Safety recommendations made following aircraft accidents at similar events have focused on protecting spectators, pilot medical fitness and aircraft maintenance.

“The (Federal Aviation Administration) has tightened airshow requirements. This will certainly raise the debate again,” said Steven Wallace, former director of the FAA’s office of accident investigations.

On Monday, officials identified the six men killed Saturday when a World War II-era bomber and a fighter plane collided and crashed in a ball of flames at the Commemorative Air Force Wings Over Dallas show. All six were experienced aviators with years of flight training, including as current and retired airline pilots and retired military pilots.

The National Transportation Safety Board is leading the investigation into why the aircraft were flying at the same altitude and in the same air space, NTSB member Michael Graham said.

The Commemorative Air Force, which put on the show, identified the victims as: Terry Barker, Craig Hutain, Kevin “K5” Michels, Dan Ragan, Leonard “Len” Root, and Curt Rowe.

All of the men were volunteers, but each had gone through a strict process of logging hours and training flights and were vetted carefully, Hank Coates, the CEO of Commemorative Air Force said at a weekend news conference.

Officials have not publicly identified which of them were piloting the aircrafts.

Hutain, of Montgomery, Texas, had been a commercial airline pilot since 1985. He started flying at the age of 10 and had logged more than 34,500 flight hours, according to his LinkedIn page.

In a recent interview with Vintage Aviation News posted on YouTube, Hutain described aviation as a “lifelong obsession” passed down from his father, a bomber pilot in World War II.

Barker was a retired pilot who had worked for American Airlines and lived in Keller, Texas. He was an Army veteran who flew helicopters during his military service.

Rowe, a member of the Ohio Wing Civil Air Patrol, was a crew chief on the B-17, his brother-in-law Andy Keller told The Associated Press on Sunday. Rowe, of Hilliard, Ohio, participated in air shows several times a year because he loved WWII aircraft, Keller said.

Root, also from Keller, was a pilot and manager for the Gulf Coast Wing of the Commemorative Air Force who worked as a contract commercial pilot, according to his LinkedIn page.

There were no reports of injuries on the ground and that can probably be attributed to a “very careful evaluation over the decades” by the NTSB and FAA to protect spectators, said former NTSB investigator and safety author Alan Diehl.

Jeff Guzzetti, a pilot who spent more than 30 years investigating aircraft accidents for the NTSB and FAA, said while much of the regulatory focus over the decades has been on protecting spectators, other recommendations have led to incremental, cumulative safety improvements in emergency response, pilot medical fitness and aircraft maintenance at air shows.

John Cudahy, president of the International Council of Air Shows, a trade group that sets air show standards, said his group and others don’t typically get many recommendations from the FAA or the NTSB following such accidents because they don’t tend to result from systemic or procedural problems, or gross negligence.

“When they do make a recommendation, we listen very attentively. We are very collaborative,” Cudahy said.

Guzzetti said he doesn’t believe there has been “any systemic degradation of safety with these air shows.”

While the ages of those who died Saturday was not immediately known, James E. Hall, who was NTSB chairman from 1994 to 2001, said the age of the pilots is an issue that must be reviewed.

The planes need more scrutiny, too, “because like the crews in these situations, the aircraft are much older.”

Graham said investigators are analyzing radar and video footage to pinpoint the exact location of the collision. Debris will be carefully examined, along with audio recordings from the air traffic control tower, pilot training records and aircraft maintenance records, he said.

Neither aircraft was equipped with a flight-data recorder or a cockpit voice recorder, separate devices referred to collectively as the black boxes, and neither were required to have those devices, Graham said.

Although rain was hampering the collection of pieces of the B-17 bomber, Graham said Monday an electronic flight display from the B-17 and a GPS navigational unit from the fighter, both damaged, will be sent to an NTSB laboratory to see if data can be recovered.

He said it’s also possible the NTSB could recommend vintage aircraft install flight data recorders.

The crash came three years after the crash of a bomber in Connecticut that killed seven, and amid ongoing concern about the safety of air shows involving older warplanes. The company that owned the planes at the Dallas show has had other crashes in its more than 60-year history.

A preliminary report from the NTSB is expected in four to six weeks, and a final report will take up to 18 months to complete.

The B-17, a cornerstone of U.S. air power during World War II, is an immense four-engine bomber that was used in daylight raids against Germany. The Kingcobra, a U.S. fighter plane, was used mostly by Soviet forces during the war. Most B-17s were scrapped at the end of World War II and only a handful remain today, largely featured at museums and air shows, according to Boeing.