"This is an ideal example of how individuals can be captured and interrogated and intelligence can be gleaned, and then we can decide what to do with them," a senior administration official said.
Warsame is in federal custody in New York, where he was flown. Currently, no terrorism suspect is being held on a U.S. ship, the officials said. But they pointedly did not foreclose the possibility of similar detentions in the future. They declined to say how often such detentions had happened.
During his interrogation, Warsame provided valuable information about activities in Yemen and links between the Yemen-based Al Qaeda in the Arabian Peninsula and Shabab, the officials said. They did not link him to any plot or attack against the U.S., but said he had "clearly served as an important conduit" between Shabab and Al Qaeda in the Arabian Peninsula, facilitating contacts and weapons transfers.
In the last year, U.S. intelligence officials have seen signs of increasing cooperation between the two organizations, based in two of the world's poorest, least-stable countries. Somalia and Yemen are linked by traditional sea trade routes across the Gulf of Aden, which Al Qaeda in recent years has been able to use for the movement of arms and fighters, according to a U.S. intelligence official who spoke on condition of anonymity because of the sensitivity of the information.
Foreign fighters from around the Arab world, drawn to the seemingly endless battles in Somalia, have traveled in recent years to Yemen and crossed the Gulf of Aden by boat to avoid being on an airline manifest, the official said.
In 2009, key leaders of Shabab in Somalia pledged allegiance to Al Qaeda in a video posted on the Internet. Al Qaeda had been trying to bring local insurgent groups in East Africa into its leadership structure for more than a decade, and had made major inroads with Shabab, according to U.S. officials.
A senior Al Qaeda operative in East Africa, Fazul Abdullah Mohammed, whom U.S. officials have labeled as the mastermind of the 1998 bombings of U.S. embassies in Tanzania and Kenya, was gunned down by Somali government security forces at a checkpoint outside Mogadishu, the capital, on June 7. Mohammed was a key link between Shabab and the top leaders of Al Qaeda, as well as the leadership of Al Qaeda's affiliate in Yemen, a U.S. intelligence official said.
"Shabab has faced serious setbacks," said Rick Nelson, a counter-terrorism expert at the Center for Strategic and International Studies, a Washington-based think tank.
The relationship between Shabab and the Al Qaeda affiliate in Yemen is one of "mutual convenience," Nelson said. Militants in Yemen have an ample supply of firearms, and the Somalis have better sources of income from piracy and kidnap-for-ransom schemes, said Nelson, citing independent research done by the center in East Africa.
David S. Cloud and Brian Bennett in the Washington bureau contributed to this report.