Polish prosecutors say a railroad traffic controller is likely to blame for the deadly head-on collision of two passenger trains Saturday.

Prosecutors said Monday the man will be charged with unintentionally causing an accident. They also are questioning another railroad worker in the crash that killed 16 people and injured more than 50 others.

Poland on Monday began two days of national mourning for the victims of the crash.

Authorities say the trains were traveling toward each other on the same track when they crashed in Szczekociny in southern Poland. One train was headed to the capital, Warsaw, from the southeastern city of Przemysl, while the other was heading south from Warsaw to Krakow.

Prime Minister Donald Tusk has called it “the most tragic train catastrophe” in Poland in recent years.