West Bengal Chief Minister Mamata Banerjee on Thursday demanded an inquiry into the derailment of five bogies of the Howrah-Dehradun Doon Express in Uttar Pradesh, stating it should be probed whether it was an 'accident or an incident'. "Since the accident has taken place during a countrywide bandh, we want this to be probed. Who has caused this mishap? It has to be seen whether this is truly an accident or an incident," Banerjee, a former railway minister, said.