The FBI sounded a warning against a possible attack in the United States because

The FBI warned a planned terror attack may occur in the United States or against U．S．interests in Yemen，as reported on Tuesday，  but U．S．  officials do not know the target or how the suspected plot would be carried out．  The FBI said the uncorroborated threat came from information discovered in  Afghanistan，where the U．S．military has been sifting through material left by al Qaeda members，and from interviews with detainees，including al Qaeda members，held at the U．S．Navy Base at Guantanamo Bay，Cuba．Rushing to put out the alert late Monday night after recently discovering the possible threat， the FBI said one or more operatives might be involved in the attack，including a Saudi-born Yemeni man．