Polish emergency services work at the site of a train crash near the town of Szczekociny, March 4 2012.
Bulgarian Foreign Minister Nikolai Mladenov has sent condolences to his Polish counterpart Radoslaw Sikorski after the train crash on March 3 near the town of Szczekociny in which 16 people died.
"On the day of national mourning, I want to express on my own behalf and on behalf of the people of Bulgaria, most sincere condolences and deepest sympathy on your bereavement," Mladenov said in a letter to Sikorski.
Rescuers worked all night on March 4 to look for for survivors and recover bodies, the Voice of America said. The March 3 crash injured at least 56 people, some seriously.
While most of the victims were Polish, an American woman was among the dead, while citizens of Ukraine, Moldova and the Czech Republic were injured.
The impact left tons of mangled metal on the tracks, with train cars overturned and piled on top of each other.
Prime minister Donald Tusk visited the crash site, calling it "the most tragic train catastrophe" in Poland in recent years. President Bronislaw Komorowski announced two days of national mourning.
One of the trains was heading to the southern city of Cracow from the capital, Warsaw, while the other was traveling north from the south-eastern city of Przemysl to Warsaw. The Cracow train was on the wrong track, a Polish railways official said, the Warsaw Voice reported.
The collision occurred on a stretch of track that was recently modernized, but officials said it was too early to speak about a cause.
Transport minister Slawomir Nowak said that train travel in Poland is still safe and that the government makes safety a priority as it improves the system.
Prosecutors launched an investigation into how the train was on the wrong track, but officials said it was too soon to draw any conclusions. They had not ruled out human error, the Warsaw Voice said.