At least six people are dead - including a girl and her mother - following the US's second aircraft tragedy in two days. The relatives and four crew members were travelling in a medical transport jet, which plummeted to the ground seconds after take off in Philadelphia on Friday at 6pm local time.It exploded into fireball, which engulfed homes. A major incident was declared. A recovery mission continues. People have been warned to stay away from north eastern Philadelphia.Debris covers city streets and homes are destroyed, witnesses said. The tragedy happened two days after a plane crash in Washington DC claimed 67 lives.Shai Gold, a spokesperson for the company which owned the Philadelphia plane - a Learjet 55 - said: "When an incident like this happens, it's shocking and surprising. All of the aircraft are maintained, not a penny is spared because we know our mission is so critical."He said the girl had just completed treatment in Philadelphia for a life-threatening condition and was returning home to Mexico. Jet Rescue, which owned the plane, said the four crew members were also from Mexico.Videos and photos taken from the scene show a massive rescue response, with dozens of fire engines moving towards the area. Other photos show an explosion as the plane went down in what is primarily a residential neighbourhood.Reports from those on the ground say there is a 'large debris field', with multiple cars on fire and the 'smell of fuel in the air'. The aircraft has been identified as a Learjet 55, a Jet Rescue Air Ambulance.Michael Schiavone, 37, was sitting at his home in Mayfair when he heard a loud bang and his house shook. When he checked his security camera, he said it looked like a missile was coming down.Mr Schiavone said: "There was a large explosion, so I thought we were under attack for a second." Flight data showed a small jet taking off from the airport at 6.06 pm and disappearing from radar about 30 seconds later after climbing to an altitude of 1,600 feet (487 metres).The plane crashed in a busy intersection near Roosevelt Mall, an outdoor shopping centre where first responders are blocking traffic and onlookers are crowded onto a street corner in the residential neighbourhood of Rhawnhurst. Philadelphia's emergency management office said roads are closed in the area.