At least 39 people were killed on Sunday night when a bus burst through a guardrail and fell nearly 100 feet into a ravine in Southern Italy. Witnesses say the "out of control" bus hit several other vehicles before breaking through the barrier and rolling down the steep hill, though it still is not clear what started the accident.

The bus was reportedly ferrying about 50 passengers, including some children, on a weekend pilgrimage to the Padre Pio shrine in San Giovanni Rotondo and was on its way back to Naples when the accident occurred. The toll of casualties is still uncertain, however, as rescuers are continuing to comb through the wreckage.

A relative of one of the survivors said they believed it may have been a blown tire that caused the driver to lose control. The driver was among those killed. Sixteen others were injured, including some in the other cars that were struck by the bus.

The crash came at the end of a week filled with transportation disasters. On Saturday, a bus carrying a church group home from camp lost control and flipped outside of Indianapolis, killing three people, including a pastor and his pregnant wife. Two people were killed, including a bride-to-be just two weeks out from her wedding, in a boat crash on the Hudson River just North of New York City on Friday night. And last Wednesday, a speeding train derailed in northern Spain after taking a corner too quickly and killed nearly 80 peopleThe driver is being charged with negligent homicide.