At least 25 people were killed after their bus veered off a slippery mountainous road in the southwestern province of Guizhou on Tuesday morning, according to the local production safety watchdog.

A 35-seat passenger bus, carrying 38 people, veered off a highway in the mountainous area of Zunyi City and plunged into a 40-meet-deep valley at about 7:40 a.m., officials with the provincial work safety supervision administration said.

Two passengers suffering serious injuries were in a critical condition, and another 11 had slight injuries, doctors said.

Initial investigation blamed the tragedy on the unusual snowy weather in the region over the past days. The road was covered with a thick layer of ice and the temperature was about minus two degrees Celsius, a sharp contrast to the usual warm winter climate.

Investigators are examining the possibility that traffic administrators should not have allowed the bus to travel in such poor weather conditions.