Passengers had to be taken off a bus in Sheffield city centre after a glass window shattered.
The number 20 bus to Hemsworth had pulled up on Arundel Gate yesterday morning at around 9am when passengers noticed that the glass had shattered.
One passenger on the bus said he believed that the window had been attacked by vandals.
However, he said it 'all happened too fast' for him to notice anyone throwing any objects at the bus.
A First South Yorkshire spokesperson said they had launched an investigation into the cause of the incident.
They said: "We can confirm that a window on one of our vehicles shattered this morning and we are trying to establish the circumstances and cause.”
In August, shots were reportedly fired at a moving bus in Sheffield in what was feared to be an escalation of attacks using bricks and stones.
A YouTube video revealed two shattered windows on the bus, which the person posting the footage claims were caused by shots fired at the number 3 travelling along Montgomery Road in Sharrow.