Officials in Dallas, Texas, are investigating the cause of a fatal mid-air collision between two historic military planes during an airshow on Saturday. The incident instantly caused the planes to crash to the ground, sending an explosion of fire and black smoke into the afternoon sky.

The collision occurred around 13:20 over the Dallas Executive Airport, about 10 miles (16 km) from downtown Dallas. A video posted on Twitter by a spectator captured the World War II-era planes, a B-17 Flying Fortress bomber and a Bell P-63 Kingcobra fighter crashing on impact.

*Video and images of the incident may be graphic to some.

The crash scene

Local news aerial footage showed the wreckage and debris at the crash scene in a grassy area of the airport. Dallas Fire Rescue confirmed that people were not injured on the ground. The number of fatalities or injuries has not been confirmed, but an official said the B-17 typically seats around four to five people and the Kingcobra has one single pilot.

The Federal Aviation Administration (FAA) provided a statement Saturday afternoon about an initial investigation.

“At this time, it is unknown how many people were on both aircraft. The FAA and the National Transportation Safety Board will investigate. The NTSB will be in charge of the investigation and will provide additional updates.”

The Allied Pilots Association (APA), the union representing American Airlines pilots, said that two former members were in command of the B-17.

"We are saddened to report that former APA members CA Terry Barker and CA Len Root were among the crewmembers lost on B-17 Flying Fortress during the Wings Over Dallas airshow today," the association said on Twitter. "Our hearts go out to their families, friends, and colleagues past and present."

The association also said it would provide professional counseling services at its headquarters in Fort Worth, TX, over the next two days.

The NTSB's investigation

Hank Coates, President of Commemorative Air Force, the company responsible for putting on the airshow, confirmed that the company owned both planes and that its fleet is flown by highly trained volunteers who are often retired pilots.

In a press conference, Coates said that he could not release any further information on the names, the number of people involved, and their status until all next of kin have been notified and the NTSB approves the release of the information. The NTSB will arrive at the scene to begin its investigation on Sunday.

Witnesses weigh in

Anthony Montoya attended the airshow with a friend and witnessed the mid-air collision.

“I just stood there. I was in complete shock and disbelief,” said Montoya to the Associated Press. “Everybody around was gasping. Everybody was bursting into tears. Everybody was in shock.”

Dallas Mayor Eric Johnson said his city experienced a "terrible tragedy."

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

The Wings Over Dallas airshow, known as America's Premier World War II Airshow, was scheduled to be a three-day event celebrating Veterans Day weekend, where guests would see more than 40 historic aircraft. Friday's show was canceled due to inclement weather. According to the airshow's website, Sunday's events will not occur.

Sources: The Associated Press, Fort Worth Star-Telegram