A bus carrying Polish pilgrims veered off a road in Croatia on Saturday and crashed into a ditch, killing twelve people.

19 of the 32 surviving passengers are reportedly in critical condition.

The Brotherhood of St. Joseph’s Catholic group organized the trip, which included three priests and six nuns. They were traveling to the Catholic shrine of Medjugorje in Bosnia.

The passengers were all adult Poles, according to the Polish Ministry of Foreign Affairs.

After a prayer service, the bus left the Polish city of Czestochowa on Friday night. The passengers were from different regions of Poland, according to the foreign ministry.

The accident occurred at approximately 05:40 local time (04:50 GMT) when their bus veered off the A4 road between Jarek Bisaski and Podvorec, northeast of Zagreb.

The justice minister and prosecutor general of Poland has ordered the Warsaw Prosecutors Office to investigate the cause of the tragedy, and two Polish ministers are en route to Croatia in the wake of the incident.

Initially, eleven fatalities were reported, but a twelfth victim died in the hospital.

“Some of the injured passengers are fighting for their lives,” said Davor Bozinovic, the Croatian minister of the interior.

Andrej Plenkovi, the prime minister of Croatia, expressed his condolences to the families of the victims, adding in a tweet that emergency services were doing everything possible to assist.

Following reports that local children witnessed a vision of the Virgin Mary in the 1980s, pilgrimages to the small town of Medjugorje are very popular in Poland.