WASHINGTON -- The U.S. Postal Service on Friday issued a warning for people to take care when opening their mail boxes, after two pipebombs exploded in two Midwest states causing minor injuries, AFP reported. The devices, which exploded as boxes were opened, were among a half dozen that had been placed in Iowa and Illinois. Two people were injured in the incidents. "The explosives were intended to detonate when they were removed from the rural mailboxes," said a postal service official in Des Moines, Iowa, Linda Jensen told reporters. "The devices are accompanied by a clear plastic bag that contains a typed letter written full of anti-government propaganda," she added. An inquiry has been opened involving the Federal Bureau of Investigation, the spokeswoman said, making an appeal to the public for information.