At least 15 people are dead and another 18 injured after a streetcar derailed in Portugal’s capital on Wednesday, officials said.

At least five people are in serious condition following the crash in Lisbon, according to the city’s communications department.

It appears the safety cable on the electric streetcar broke, causing the car to derail, the department said, based on preliminary information.

The mayor of Lisbon declared a three-day period of mourning.

“I offer my sincere condolences to all the families and friends of the victims. Lisbon is in mourning,” Mayor Carlos Moedas said in a statement.

Portugal’s Prime Minister’s Office also declared a national day of mourning for Thursday and said it is in contact with local officials.

The incident remains under investigation.

The tram cabin that derailed can hold up to 40 people. The famed streetcar, known as the Elevador da Gloria, is a funicular that travels up and down a steep hill.

“It is with sadness that I learned of the derailment of the famous ‘Elevador da Glória,'” European Commission President Ursula von der Leyen said in a statement. “My condolences to the families of the victims.”

This is a developing story. Please check back for updates.