What is West Bengal's Atta Chawal Scam which led to an attack on ED officials

A mob charged on on-duty ED officials in West Bengal today while the team was on their way to conduct a raid on TMC leader Shahjahan Sheikh’s house. The raid was in connection to the ration scam worth thousands of crores. Find out what is Bengal's atta chawal scam which escalated into a serious law and order situation in the state.