A blast at a chocolate factory in the U.S. state of Pennsylvania killed at least five people and left six others missing. Emergency services said Saturday as search workers continue to look through debris for survivors.

The blast occurred on Friday at the R.M. Palmer Company plant in West Reading, a town about 60 miles (97 km) northwest of Philadelphia, the Pennsylvania Emergency Management Agency (PEMA) said.

PEMA said an investigation would be conducted to determine the cause of the explosion and that an initial incident report included a reference to a gas leak.

Seven other people were transported to local hospitals with injuries, it added. PEMA did not provide details on search and rescue operations.

Images on social media of the explosion's aftermath showed a vast pile of debris and massive flames letting off large plumes of thick black smoke in a downtown area surrounded by other buildings.

The R.M. Palmer Company was founded in 1948. According to its website, it employs 850 people and creates 500 unique chocolate and candy products.

The company did not immediately respond to a request for comment.