Mail workers discovered a substance in a letter sent to the Pentagon that has initially tested positive for the lethal poison ricin, defense officials are reporting. The mail facility is said to be separate from the Pentagon structure, and the ricin never actually entered the main building. As The Daily Beast previously reported, ricin is a lethally toxic compound made by castor beans and has no antidote. Because of its potency, it has been used in a series of attempted attacks on U.S. officials, including former President Barack Obama and Sen. Roger Wicker (R-MS), and was cited on a list of useful poisons in an al Qaeda document. The FBI and the Pentagon Force Protection Agency are responding at the scene.