ST. LOUIS, Mo. A Federal Express truck exploded Tuesday along I-270 in St. Louis, scattering debris all over the road and forcing officials to close the highway near the Illinois state line. Nobody was injured, including the two drivers who were in the truck at the time of the explosion, Federal Express spokeswoman Sandra Munoz said. Officials were trying to determine what caused the blast, which demolished the trailer of the 18-wheeler but left the cab undamaged. Authorities suspected that a combustible package may have exploded inside the trailer. An explosives expert told Fox News the blast did not appear to be the result of a bomb. He said the debris was dispersed over a relatively small area, and that a bomb would likely leave a large hole in the ground at the scene. The FBI dispatched agents to the scene, but the bomb squad was not called to investigate. The trailer blew up on the northbound side of the highway, near the Bellfontaine exit on the north end of St. Louis County.