The U.S. Justice Department said the teenager was ordered detained by the federal judge. The New York Times reported the judge found a telephone conversation with the youth's father in Somalia, who claimed the suspect was underage, not credible.
The youth identified by the department as Abduwali Abdukhadir Muse told the judge through an interpreter, "I don't have any money," the New York Daily News reported. The suspect was brought to New York because the FBI's Manhattan office handles crimes against U.S. citizens in Africa.
Muse is charged with piracy and four related counts. If convicted on the piracy charge alone, he could face life in prison, the department said.
The courtroom was closed while officials argued about his age, which in the early stages seemed to be in some dispute.
The thin, 5-foot-2-inch suspect's court-appointed lawyer, Phillip Weinstein, said the defendant's father told him Muse is only 15. The Justice Department released a statement saying Muse "is over 18 years old." The suspect's mother said he is only 16.
But U.S. Magistrate Andrew Peck said, "The court has determined that Mr. Muse is not a juvenile and we can proceed in open court" after the closed-door proceedings.
Officials say the teenager is one of four pirates who attempted to hijack the cargo ship April 8. Phillips offered himself as a hostage and was freed April 12 by U.S. Navy SEALs who shot and killed three pirates during the rescue.
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