A medical helicopter crashed in a mountainous area near Porto, Portugal’s second biggest city, with all four onboard feared dead, local media and the country’s INEM emergency service reported on Saturday.

State broadcaster RTP and daily Correio de Manha reported the helicopter had crashed, killing two pilots, a nurse and a doctor onboard. INEM said a rescue operation was under way but the helicopter had still not been found.

The INEM helicopter went missing at around 6:30 p.m. on Saturday after dropping off a 76-year-old patient suffering from cardiac problems at a hospital in Porto.

A spokesperson at the Valongo fire station in the region said 200 rescue workers were searching for the helicopter.

According to INEM, the helicopter was on its way back to its base in the district of Braganca when it crashed in bad weather.