LISBON, Portugal (AP) — Portuguese authorities say four people have died when a helicopter in the country's medical emergency service crashed.

Portugal's National Institute of Medical Emergencies says Sunday that the helicopter went down Saturday in the Serra de Pias hills east of Porto while it was returning to base after transporting a patient. The two pilots, a doctor and a nurse died in the crash.

The service says the cause of the crash is under investigation, but added it came as the area experienced "adverse weather."

The service did not reveal the identities of the four victims. Spanish media reported that the doctor was a Spaniard.