Spain declared three days of mourning Thursday as investigators continued probing what caused a passenger jet to crash on takeoff in Madrid, killing 153 people.

Passengers said they saw flames and then heard an explosion moments before Spanair Flight JK5022, carrying 172 people, crashed at Madrid's Barajas Airport Wednesday afternoon.

There were 19 survivors, including two infants, Development Minister Magdalena Alvarez said. All were being treated at a hospital, Alvaraez said.

The severity of the injuries varies, but many of the injured have been treated for burns, Spanish Red Cross spokeswoman Olivia Acosa said.

The United States National Transportation Safety Board is sending an investigation team to Madrid because the aircraft, a twin-engined MD-82, is American-made, NTSB spokesman Keith Holloway said.

The Spanish flag at the Olympics was due to fly at half mast, while the Spanish football team wore black armbands for its friendly match against Denmark last night.