Police spokesman David Procopio says an explosion that blew out the BoltBus' windows Monday is believed to have been caused by a buildup of hot gases in the burning bus. He says investigators determined no passengers were carrying any hazardous material.
The bus made a maintenance stop in Connecticut during its trip from New York. It caught fire on the Massachusetts Turnpike in suburban Newton. The driver and the 47 passengers escaped unhurt.
The highway closed briefly. The passengers were taken on two other buses to Boston.
Dallas-based BoltBus parent company Greyhound Lines Inc. said Tuesday it's investigating and cooperating with authorities.
Federal transportation officials also will investigate.