(CNN) -- Hundreds of pieces of mail fluttered onto interstates in eastern Missouri Sunday after the back door of a contractor's semi carrying mail for the U.S. Postal Service popped open.

The mail was scattered along 70 miles of highway near St. Louis, according to Postal Inspector Dan Taylor, who said the tractor-trailer was carrying mostly statements and bills bound for the West Coast. He added that he didn't think any personal mail was lost.

The truck had just left a St. Louis-area distribution center and was headed to a Memphis, Tennessee, distribution point, Taylor said.

Police and postal authorities spent hours along the highway picking up mail Sunday afternoon.

The Postal Service is investigating how the incident happened so it can be prevented in the future, Taylor said.