At least 43 people are dead and 57 have been hospitalized after a passenger train and a freight train collided on Tuesday night near the city of Larissa in Greece, according to local authorities, writes NPR.

Photos and video of the scene from Wednesday morning showed rescuers still maneuvering through twisted wreckage, searching for survivors.

By midday, with hope fading, investigators had begun turning to what caused this crash. The country’s prime minister said in a televised address that “human error” was largely to blame.

According to the national rail operator, the passenger train was carrying roughly 350 people at the time of the collision, traveling at high speed from Athens to Thessaloniki, Greece’s second-largest city.

The Associated Press reported that many of those passengers were students returning from Carnival, a three-day festival that precedes the religious season of Lent. The freight train was likely carrying construction material, such as heavy steel plates, according to Greece’s public media agency, ERT.