Four people died after a helicopter-ambulance crashed in northern Portugal, according to emergency services agency INEM.

The aircraft came down in Valongo district, killing all onboard: a Spanish doctor, nurse and two pilots. Local officials said that the cause of the accident was not yet known.

On Saturday, December 15, the helicopter vanished from radar screens at around 6:30 p.m. (1830 GMT), civil protection officials said. The wreckage of Agusta A109S, operated by British company Babcock, was found early Sunday morning.

The helicopter had dropped off a seriously ill patient at Porto and was flying back to its base in Macedo de Cavaleiros when it came down, INEM announced.

This is the first serious accident since the emergency services of Portugal started using helicopters in 1977.