More than 100 people were working in both R.M. Palmer chocolate factory buildings at the time.
Employees reported the smell of gas an hour before the blast but no one gave an evacuation order.
WATCH: Survivor pulled from rubble of chocolate factory explosion
Survivor pulled from rubble of chocolate factory explosion in West Reading
The NTSB found that R.M. Palmer's emergency preparedness plan did not include procedures regarding natural gas emergencies.
The company has since updated its employee manuals and added natural gas detectors in other buildings.
R.M. Palmer said in a statement it is committed to the health and safety of all workers, and that it is cooperating with the federal investigation.