Al-Libi's al-Qaida ties date back to the terrorist group's early years, according to court documents. That would make him a valuable source of information about the group's history.
It's unclear whether he could offer fresh intelligence on the group, the core of which has been battered and fragmented.
Al-Libi has longstanding health issues and will get medical testing while in custody to determine whether he needs treatment, U.S. officials said. Where exactly al-Libi is being held and where that testing would take place is unclear.
Al-Libi, whose full name is Nazih Abdul-Hamed al-Ruqai, used to be on the FBI's list of most wanted terrorists. His family has denied he was in al-Qaida.
Known as one of al-Qaida's early computer experts, al-Libi is believed to have used an early-generation Apple computer to assemble surveillance photographs in Kenya before a bombing there killed more than 200. That information was presented to Osama bin Laden, who approved the bombing, a former federal law enforcement official has said.
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Associated Press writers Lara Jakes and Tom Hays contributed to this report.