Rescuers searched Wednesday through the burned-out wreckage of two trains that slammed into each other in northern Greece, killing at least 36 people and crumpling several carriages into twisted steel knots.

Transport Minister Kostas Karamanlis resigned, saying he felt it was his "duty" to step down "as a basic indication of respect for the memory of the people who died so unfairly."

The cause of the crash near the Vale of Tempe, a river valley about 235 miles north of Athens, was not immediately clear, but the stationmaster in the nearby city of Larissa was arrested Wednesday. The police did not release his name. Another two people have been detained for questioning.

It's unclear at what speed the passenger train and the freight train were travelling when they ran into each other just before midnight Tuesday, but survivors said the impact threw several passengers through the windows of train cars. State broadcaster ERT quoted rescuers saying they found some victims' bodies 30-40 meters (100-130 feet) from the impact site.

Passengers say train crash was like an explosion

A teenage survivor who did not give his name to reporters said that just before the crash he felt sudden braking and saw sparks — and then there was a sudden stop.

"Our carriage didn't derail, but the ones in front did and were smashed," he said, visibly shaken. He used a bag to break the window of his car, the fourth, and escape.

Stefanos Gogakos, who was in a rear carriage, said the crash felt like an explosion, and he could see flames at the front of the train.

"The glass in the windows shattered and fell on top of us," he told ERT. "My head hit the roof of the carriage with the jolt. Some people started to climb out through the windows because there was smoke in the carriage. The doors were closed but in a few minutes train staff opened them and we got out."

Multiple cars derailed and at least one burst into flames.

"Temperatures reached 1,300 degrees Celsius (2,372 Fahrenheit), which makes it even more difficult to identify the people who were in it," fire service spokesperson Vassilis Varthakoyiannis said.

Wreckage makes rescue efforts difficult

On Wednesday, rescuers turned to cranes and other heavy machinery to start moving large pieces of the trains, revealing more bodies and dismembered remains.

"There were many big pieces of steel," said Vassilis Polyzos, a local resident who said he was one of the first people on the scene. "The trains were completely destroyed, both passenger and freight trains."

Rescuer Lazaros Sarianidis told ERT that crews were "very carefully" trying to disentangle steel, sheet metal and other material that was twisted together by the crash. "It will take a long time," said Sarianidis.

Greece's firefighting service said some 76 people were hospitalized, including six in intensive care.

More than 200 people who were unharmed or suffered minor injuries were taken by bus to Thessaloniki, 80 miles to the north. Police took their names as they arrived, in an effort to track anyone who may be missing.

Eight rail employees were among those killed in the crash, including the two drivers of the freight train and the two drivers of the passenger train, according to Greek Railroad Workers Union President Yannis Nitsas.

Greece goes from Carnival to mourning

Many of the 350 people aboard the passenger train were students returning from Greece's raucous Carnival, officials said. This year was the first time the festival, which precedes Lent, was celebrated in full since the start of the pandemic in 2020.

The government declared three days of national mourning from Wednesday, while flags flew at half-staff outside all European Commission buildings in Brussels.

Visiting the accident scene, Greek Prime Minister Kyriakos Mitsotakis said that the government must help the injured recover and identify the dead.

"I can guarantee one thing: We will find out the causes of this tragedy and we will do all that's in our power so that something like this never happens again," Mitsotakis said.

Greek President Katerina Sakellaropoulou broke off an official visit to Moldova to visit the scene, laying flowers beside the wreckage.

Pope Francis offered his condolences to the families of the dead, in a message sent to the president of the Greek bishops conference on his behalf by the Vatican's secretary of state,

The pontiff "sends the assurance of his prayers to everyone affected by this tragedy," the message said.