It's Friday on Rikers Island, time for weekly worship for nearly a quarter of the city jail's 14,000 inmates. The men, Muslims, file quietly into a classroom of white cinderblock that serves as their mosque. Incense burns to chase away a sour smell from the hall, as the inmates sit quietly on sheets stamped "Department of Corrections" covering the linoleum floor. Imam Menelik Muhammad is delivering the day's sermon. As he stands beneath a Quranic prayer on the wall facing Mecca, he urges the prisoners to reform. "You will not be considered a Muslim," he admonishes, "unless people are considered safe from your hands and your tongue." Across the United States, tens of thousands of Muslims are practicing their faith behind bars. But the Sept. 11, 2001, attacks have brought new scrutiny to Muslim inmates, many of whom are black men focused on surviving incarceration. The FBI calls prisons "fertile ground for extremists." The reality is harder to read: Those on opposing sides have such divergent views they seem irreconcilable. Who's right matters not only for national security, but for the development of American Islam itself, which is struggling to be accepted alongside the major faiths in the United States.