Assistant U.S. Attorney Sean S. Buckley said the men made statements to officers in the United Kingdom that may be part of the evidence at a trial he estimated would last up to three months.
Two other men brought from Britain were arraigned in Connecticut on Saturday on terrorism charges.
Mustafa became well-known in the 1990s as his Finsbury Park Mosque in London became a training ground for extremist Islamists including Sept. 11 conspirator Zacarias Moussaoui and attempted "shoe bomber" Richard Reid. He had been jailed since 2004 in Britain on separate charges.
Traci Billingsley, a U.S. Bureau of Prisons spokeswoman, said she cannot provide specific information about individual inmates.
"In general, if an inmate arrives at any of our facilities with a prosthetic that we believe could pose a danger, it would not be permitted inside," she said, adding that the inmate would be medically evaluated to determine whether other accommodations or devices would be appropriate.
John N. Billock, head of the Orthotics & Prosthetics Rehabilitation Engineering Centre in Warren, Ohio, and a pioneer in the field, said a hook for a hand would "definitely be considered a weapon."
"You could brutalize somebody with it," he said. "You can put somebody's eyes out or knock out their teeth."
He said hooks are typically made of stainless steel or aluminum. The price of prosthetics in place of hands can range from $15,000 to $100,000, he said.
Mustafa is being held prior to trial in the same federal lockup where a prison guard lost an eye and was left brain-damaged when he was stabbed with a sharpened comb in 2000 by a terrorism defendant awaiting trial in the embassy bombings plot.
Mamdouh Mahmud Salim is serving a life sentence after pleading guilty.