The National Transportation Safety Board (NTSB) released a preliminary report Wednesday on the November air show accident at Dallas Executive Airport that killed six people.

During the “Wings Over Dallas” airshow on November 12, a B-17 Flying Fortress and P-63 King Cobra crashed during the course of the show around 1:20 p.m., as reported by The Dallas Express.

Although the NTSB said that a full investigation could take more than a year, the preliminary report highlighted the agency’s initial findings.

Radio transmissions from the airshow and Automatic Dependent Surveillance-Broadcast (ADS-B) show that the air boss, who was directing the show, asked the King Cobra plane, which was in the midst of a three-plane formation with other historic flying warplanes, and the Flying Fortress, which was in a five-plane formation of historic bombers, to proceed southwest of the runway before returning to the display area.

The fighter formation was to move into a trail formation, fly in front of the bomber formation, and then head toward the 500-foot show line. The bombers were told to fly to the 1000-foot airshow viewing line, located behind where the audience was viewing the show.

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According to the NTSB, altitude maneuvers were not discussed before take-off or while the planes were in the air. While the fighter formation approached the formation area, the King Cobra, which was on the left bank, collided with the Flying Fortress behind the wing section.

The planes broke apart in flight and collapsed over Highway 67 and the Dallas Executive Airport.

Five people, including the pilot, co-pilot, and three crew members, were killed in the Flying Fortress, and the pilot of the King Cobra was killed.

The victims were later identified as Terry Barker, Maj. Curtis Rowe, Craig Hutain, Len Root, Dan Ragan, and Kevin “K5” Michels, as reported by The Dallas Express.

Jon Kettles, an aviation attorney, told WFAA he believed the accident resulted from an error. According to Kettles, it appeared the bomber was either ahead of its planned path or the P-63 was behind, so when the P-63 turned to get into position, it crashed into the bomber.

“There was obviously a breakdown in the procedure as well as the execution,” Kettles said. “We work in a three-dimensional world. They should’ve had both horizontal separation. In this case, they were using distance from the audience, as well as vertical separation and that didn’t happen.”

The Commemorative Air Force, the organization behind the show, released the following statement Wednesday: “We are continuing to work with NTSB and are grateful for their diligence in looking into anything that could have been a factor to cause the accident. Until the NTSB’s final report comes out, we cannot speculate about any cause or causes that may have led to the accident. We will continue to post any updates on our website.”

The wreckage of both airplanes was retained by NTSB for further examination.