President Barack Obama on Sunday ordered a review of government procedures for screening airline passengers and for tracking individuals suspected of terrorist ties.
The administration also said it was tightening security procedures, which airlines and passengers say is already causing delays on international flights.
The incident comes on the heels of nearly a dozen terrorism probes and alleged plots to come to light in recent months. The string of cases highlights the difficulty, more than five years after the 9/11 Commission called for better communication between intelligence, law-enforcement and security agencies, of identifying relevant information that could stop a terrorist attack.
One U.S. official briefed on the inquiry said investigators are still trying to determine whether the suspect's claims of links to al Qaeda in Yemen are accurate, and how strong those ties are.
Delta Air Lines Inc., which acquired Northwest last year, said the crew of Sunday's Flight 253 asked authorities to meet the plane upon landing in Detroit because of a "verbally disruptive" passenger. All 257 passengers and 12 crew members got off the plane safely.
The Homeland Security Department said Sunday evening that "indications at this time are that the individual's behavior is due to legitimate illness, and no other suspicious behavior or materials have been found."
In the aftermath of the attempted Christmas bombing, federal agents are working with authorities in Britain, the Netherlands, Yemen, and Nigeria to determine whether Mr. Abdulmutallab was part of a wider plot. Mr. Abdulmutallab told investigators he had affiliations with al Qaeda operatives in Yemen, who gave him the device and detonation instructions to blow up the plane, according to U.S. officials.
Law enforcement officials said there is no evidence yet to indicate that Mr. Abdulmutallab was part of, or in contact with, any terror cell in the U.S. or the U.K., and that early evidence indicates he was radicalized through contacts with extremists via the Internet.
Federal prosecutors are expected on Monday to request a judge's permission to obtain DNA from Mr. Abdulmutallab to compare with DNA found on remains of the device taken from the aircraft.
The Federal Bureau of Investigation said the device contained the explosive PETN, which convicted "shoe bomber" Richard Reid used in his 2001 attempt to bring down a trans-Atlantic flight.
U.S. officials say the accused man's father, a prominent banker in Nigeria, had warned officials at the U.S. embassy in Lagos, Nigeria, in recent weeks that he feared his son had been "radicalized" during trips outside the West African country.
The father's concerns about his son weren't specific, nor did they point to any imminent threat against the U.S., according to a U.S. official. But they were enough for U.S. authorities to add his name to a broad terrorism database, called Terrorist Identities Datamart Environment. People on the list are not precluded from boarding flights to the U.S. Mr. Abdulmutallab wasn't added to more sensitive databases, such as the so-called "no fly" watch list, that would have flagged him for additional screening or barred him from boarding a U.S.-bound flight.
In June 2008, the U.S. Embassy in London had issued a multiyear, multientry tourist visa to Mr. Abdulmutallab, when the Nigerian national was a student living in the U.K., said a U.S. official. Mr. Abdulmutallab later left the U.K. and traveled to Dubai and Yemen. He was denied entry to the U.K. in May 2009 by border officials who said the school he proposed to attend wasn't legitimate.
Rep. Peter King of New York, the top Republican on the House Homeland Security Committee, said in an interview that briefings from U.S. security officials indicate that "the government definitely knew about [the alleged attacker]. They had a file on him. The question now is why wasn't he on the no-fly list."
According to a federal criminal complaint, Mr. Abdulmutallab boarded a Northwest flight, shared with Dutch Airline KLM, in Lagos on Thursday, then transferred to the Northwest airliner Friday in Amsterdam, bound for Detroit. He had a device attached to his body, according to the criminal complaint.
As the Airbus 330-300 carrying 289 people was approaching Detroit, Mr. Abdulmutallab went to the restroom for about 20 minutes. On returning to his seat, he stated that his stomach was upset, and he pulled a blanket over himself, according to the complaint. As the plane was heading for a landing at Detroit Metropolitan Airport, the complaint alleges, Mr. Abdulmutallab set off the device, causing a fire. He was subdued by passengers.