Officials said one more passenger died later in a hospital and that the bus was carrying 43 people.
Croatia’s state HRT television reported that about 30 people were injured, many seriously. It said the most likely cause of the crash was the driver falling asleep.
The broadcaster showed video of a smashed blue bus in a ditch next to the highway. The bus was travelling in the direction of Zagreb, Croatia’s capital.
Polish Prime Minister Mateusz Morawiecki said the bus was taking pilgrims to the Catholic shrine in Medjugorje, a town in southern Bosnia. The shrine is Europe’s third-most popular pilgrimage destination after Lourdes and Fatima, although the Vatican has not verified any of the reported miracles that witnesses claimed to have seen there.
“This morning, I spoke about the details of the tragedy with Croatian Prime Minister Andrej Plenkovic, who assured the full support of Croatian medical services,” Morawiecki said on Facebook.
“I recommended our consular services to organise a support organisation for the families of accident participants.”
Croatian Interior Minister Davor Bozinic said the bus originated from “a place near Warsaw” and that “according to some information,” it was carrying pilgrims to Medjugorje.
The accident happened at 5:40 a.m. local time (3:40 a.m. GMT) some 50 kilometers (30 miles) north of Zagreb, on the A-4 highway, which is busy during the peak of the tourist season.
Rescue teams were sent to the location of the accident. An investigation into the accident was ongoing, Croatian media reported.