U.S. safety board opens probe into Pennsylvania chocolate factory blast that killed seven

The National Transportation Safety Board (NTSB) on Tuesday opened an investigation into Friday's natural gas explosion and fire at a Pennsylvania chocolate factory that killed seven.

The NTSB said it was investigating the blast at the RM Palmer Company Chocolate factory in West Reading. Images on social media of the explosion's aftermath showed a pile of debris and flames letting off large plumes of thick black smoke in a downtown area surrounded by other buildings.