The National Transportation Safety Board has opened an investigation into the deadly explosion and fire at a chocolate factory in a small Pennsylvania town last Friday, the agency said Tuesday. The cause of the explosion, which killed seven people, remains unclear. An initial tweet from the NTSB identified the incident as a “natural gas explosion and fire,” but a spokesperson later clarified that it was an “accident [that] involved a pipeline that transported natural gas,” according to a reporter for CBS Philadelphia. In addition to NTSB’s probe, the explosion at the R.M. Palmer factory in West Reading is being investigated by a number of other agencies, including the Pennsylvania State Police and the Occupational Safety and Health Administration. Several people were also injured in the blast, including Betty Wright, a woman who lives adjacent to the factory. Wright has alleged in a lawsuit since filed against R.M. Palmer that the explosion lifted her off her feet and threw her across the room, “causing severe and permanent injuries.”