At least 11 people were killed and four others injured in an explosion in a firecracker factory at Pingla in West Midnapore district, the police said.   "So far 11 bodies were recovered. Four others were injured and hospitalised,' district Superintendent of Police Bharati Ghosh said.   When asked about the reason for the blast, she said, "We are looking into it. Probably something went wrong during manufacturing of crackers.'   Police said all the victims were staff members of the firecracker factory where the blast occurred.