Portuguese officials focused Thursday on establishing the causes of the crash of a Lisbon streetcar popular with tourists that killed at least 16 people and injured 21, five of them seriously.

“The city needs answers,” Lisbon Mayor Carlos Moedas said in a televised statement about Wednesday's derailment.

The downtown streetcar, which is known as Elevador da Gloria and is a major attraction for tourists packing the Portuguese capital during the summer season, came off the rails during the evening rush hour.

The crumpled wreckage was cordoned off as investigators sifted through the wreckage, took photographs and pulled up a metal cable from beneath the rails that climb one of Lisbon's steep hills.

Police, public prosecutors and government transport experts are investigating the causes, Prime Minister Luis Montenegro said during a joint news conference with the mayor.

The company that operates the streetcar service, Carris, said that it has also opened its own investigation. It said that scheduled maintenance had been carried out.

As well as those, the mayor said he would ask for an investigation from an outside independent body.

The electric streetcar, technically called a funicular, is harnessed by steel cables, with the descending car helping with its weight to pull up the other one. The car can carry more than 40 people, seated and standing.