Three Aviation Disasters In Just Days: 

F-35 Stealth Jet Crashes In Alaska

Reagan Tower Reportedly Understaffed When Army Helicopter Collided With Passenger Jet

Private Jet Crashes In Northeast Philadelphia

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Update (0850ET):

A Learjet 55 operated by Jet Rescue Air Ambulance crashed into a shopping center in Northeast Philadelphia on Friday night, killing all on board. The impact triggered a massive fireball that set homes and vehicles ablaze.

The New York Times reported that the Learjet 55 had six people on board, all of whom perished upon impact: 

The Learjet 55 was transporting a young female patient from Philadelphia, where she had completed treatment, to her home in Mexico, said Shai Gold, a spokesman for Jet Rescue Air Ambulance, which owns the plane. She was accompanied by her mother, and there were also two pilots, a doctor and a paramedic on board, he added. The patient had been treated at Shriners Children's in Philadelphia, said the hospital's spokesman, Mel Bower. All six people on the plane were Mexican, Mexico's Foreign Ministry said.

The twin-engine jet departed from Northeast Philadelphia Airport and was en route to Springfield-Branson National Airport in Missouri when it crashed just minutes into the flight.