WASHINGTON, Nov. 17 — An exclusive NBC news investigation reveals that, over the last decade, U.S. immigration officials repeatedly allowed suspected terrorists to become U.S. citizens, even when they were under FBI investigation — and it continued after 9/11.NBC NEWS has learned that Ahmed Hamade is, according to a confidential government report, an alleged associate of the Hezbollah terrorist group. Last year, even as the FBI had Hamade under surveillance in New Jersey, another federal agency mistakenly gave him the highest honor bestowed on an immigrant — the gift of American citizenship and the legal rights that go with it.