Sixteen of the 22 workers on the bus were hospitalized following Monday's crash and one remains in critical condition. Police said Tuesday the driver wasn't certified to drive a vehicle with air brakes.
Officer Mark Menke says it appears the crash was caused by a malfunctioning air compressor.
The bus is owned by Del Val Staffing. A phone message left for a company spokesman was not immediately returned Wednesday.
The workers were returning to Philadelphia from a New Jersey snack food plant when the bus crashed at the bottom of an Interstate 95 offramp.
Police have not released the driver's name.
