A Boeing B-17 Flying Fortress and a Bell P-63 Kingcobra collided and crashed at the Wings Over Dallas air show around 1:20 a.m. Saturday, according to the Federal Aviation Administration.   

  “At this time, it is unknown how many people were on board both aircraft,” the FAA said in a statement.   

  Authorities responded to the incident at Dallas Executive Airport, Jason Evans with Dallas Fire Rescue told CNN on Saturday.   

  There are currently more than 40 fire rescue units on the scene, the agency’s active incidents page shows.   

  The Commemorative Air Force identified both aircraft as being out of Houston.   

  “We currently have no information on the condition of the flight crews as emergency responders work the accident,” the group said in a statement, adding that it was working with local authorities and the FAA.   

  The FAA and the National Transportation Safety Board will investigate the crash.  The NTSB will be responsible and is expected to provide additional updates.   

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

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

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

  Debris from the crash fell onto the southbound 67 Freeway, CNN affiliate WFAA reported.  The southbound and northbound lanes of the freeway were closed after the incident, the Dallas Police Department said.   

  The B-17 was part of the Air Force Memorial collection, nicknamed the “Texas Raiders,” and was hung in Conroe, Texas near Houston.  It was one of approximately 45 complete surviving examples of the model, only nine of which were seaworthy.   

  The P-63 was even rarer.  About 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 in the early 1960s. About 3,300 P -63 were manufactured by Bell Aircraft between 1943 and 1945 and were used primarily by the Soviet Air Force in World War II.   

  This is a developing story.