Portugal’s prime minister, Luís Montenegro, has promised a swift and thorough investigation to determine the causes of the funicular railway crash that killed 16 people and injured 21 others in Lisbon, describing the incident as “one of the biggest tragedies in our recent history”.

Public prosecutors are already looking into the crash, which happened on the Portuguese capital’s famous Glória funicular just after 6pm on Wednesday. The city’s mayor has also asked Carris, the municipal public transport company that operates the service, to investigate.

All those killed when one of the funicular’s two cars derailed and crashed into a building were adults, according to Margarida Castro Martins, the head of the city’s civil protection agency.

She said the victims’ families would be informed before any names or nationalities were released, but confirmed that those injured in the crash included Portuguese people as well as two Germans, two Spaniards and one person each from Canada, Cape Verde, France, Italy, Morocco, South Korea and Switzerland.

Carris later said that one of its workers, a brake operator, had died in the crash.

The injured people, five of whom were seriously hurt, were being treated in several hospitals in the Lisbon region. One of the most badly injured victims died in hospital overnight, raising the initial death toll from 15 to 16.

Earlier figures from the civil protection agency had suggested 17 people had been killed, but the toll was corrected on Thursday afternoon.

Teams of pathologists from the national forensics institute worked through the night on postmortems, officials said. The Portuguese authorities suspended Lisbon’s other funiculars for safety checks after the derailment.

Speaking on Thursday afternoon, Montenegro called the crash a “tragic accident that transcends borders and a pain that knows no nationality”, and promised a quick investigation to establish what had gone wrong.

“The competent authorities will swiftly carry out the necessary investigations to determine the cause of this tragic accident,” he said. “We will determine all responsibilities with a sense of respect for all those who suffered and are suffering the effects of this accident.”

The two vehicles on the funicular railway, known as the Elevador da Glória, go up and down a steep hill in central Lisbon in tandem, with one going the opposite way to the other.

The leader of the Fectrans union, Manuel Leal, told local TV that workers had complained that problems with the tension of the cable that hauls the carriages had made braking difficult, but that it was too early to say if that had been the cause of the crash.

Footage showed the wreckage of the yellow-and-white car lying on its side in the narrow street it ran along. Its sides and top were crumpled, and it appeared to have crashed into a building where the road bends. Parts of the vehicle, made mostly of metal, were crushed.

Witnesses told local media that the streetcar had careened down the hill, apparently out of control. “It hit the building with brutal force and fell apart like a cardboard box,” Teresa d’Avó told the TV channel SIC.

Another witness, Bruno Pereira, told CNN Portugal that the car at the bottom of the hill had suddenly jerked off the track and on to the pavement, just before the other one came hurtling down, “and struck the walls like a rock”.

“The other tram – which counterbalances this one – arrived, completely out of control, crashing into the walls, people screaming. It hit the last time at the curve between Calçada da Glória and Rua da Glória and stopped with a loud crash,” he said.

“There was panic. We all ran, everyone there. Some people tried to pull people out from under the tram … There were cobblestones everywhere, and the metal grooves, on which the car ran, were all popped out.”

Lisbon’s mayor, Carlos Moedas, said there were no words to describe the pain the city was feeling. “We’re gathering all the information to determine who is responsible. The city needs answers,” he said.

As Portugal began a day of national morning on Thursday, the president, Marcelo Rebelo de Sousa, expressed hope that authorities would soon establish the cause of the crash.

The Glória line, which carries about 3 million people annually, is used by tourists and residents. Its two cars, each capable of carrying about 40 people, are attached to opposite ends of a haulage cable, with traction provided by electric motors on the cars.

Carris said “all maintenance protocols” had been carried out including monthly and weekly service programmes and daily inspections.

It also paid tribute to the worker who was killed, André Marques, describing him as a dedicated, kind and happy professional.

Pedro Sánchez, the prime minister of neighbouring Spain, said he was saddened by the “terrible accident”. “We send all our love and solidarity to the families of the victims and to the Portuguese people at this difficult time,” he said. “We also hope that those who have been injured will get better quickly.”

Italy’s foreign minister, Antonio Tajani, said he had met his Portuguese counterpart and expressed his solidarity with the victims.

The European Commission president, Ursula von der Leyen, expressed her sadness and offered her condolences to the families of the victims.

A spokesperson for the UK Foreign Office said it was in touch with the local authorities and stood by “to provide consular assistance if there are any affected British nationals”.

Portugal, and Lisbon in particular, has experienced a tourism boom in the past decade, with visitors cramming into the popular city centre in the summer months.

Reuters and the Associated Press contributed to this report