U.S. President Barack Obama said he "will not rest" until those behind the attempted bombing of an airliner are caught and prosecuted, while an al Qaeda-linked group claimed responsibility.
A Nigerian suspect, Umar Farouk Abdulmutallab, has been taken into custody after the incident, which took place aboard a U.S. airliner on Christmas Day.
"We will not rest until we find all who were involved and hold them accountable," Obama said Monday in Hawaii, in his first public remarks on the attempted attack.
"This was a serious reminder of the dangers that we face and the nature of those who threaten our homeland."
Al Qaeda in the Arabian Peninsula claimed it was behind the thwarted bombing. In an Internet statement, the group alleged that Abdulmutallab co-ordinated with members of the group, an alliance of militants based in Saudi Arabia and Yemen.
There's no word on the credibility of the claim. However, Abdulmutallab, is reported to have told authorities that he'd been given explosives and instructions by al Qaeda operatives in Yemen.
Officials confirmed Monday that Abdulmutallab lived in Yemen for a period of time.
Abdulmutallab was placed on the U.S.'s lowest risk watch list in November 2009, after his father expressed concerns to authorities about the behaviour of his son. People on the low-risk watch list are still allowed to be on flights transiting through the country.
Obama said he will carefully review the country's watch-list system.
"It's absolutely critical that we learn from this incident and take the necessary measures to prevent future acts of terrorism," he said.
Earlier, White House spokesman Robert Gibbs said the government will investigate its systems for placing suspicious travellers on watch lists and for detecting explosives before passengers board flights.
U.S. politicians questioned how a man flagged in a terrorist watch list could manage to get explosives on to a passenger jet.
Sen. Joe Lieberman, the chair of the Senate Homeland Security and Governmental Affairs Committee, said the committee would hold a hearing in January.
British watch list in question
British investigators are also looking into the situation. British Home Secretary Alan Johnson said police and security services are looking at whether Abdulmutallab was radicalized in Britain.
"We don't know yet whether it was a single-handed plot or (there were) other people behind it -- I suspect it's the latter rather than the former," Johnson told the BBC.
Britain's government said Monday Abdulmutallab was also placed on a U.K. watch list back in May 2009, after he was refused a student visa.
However, his U.S. visa, which was issued in June 2008 and good for two years, was not revoked.
Abdulmutallab was placed on the list known as Terrorist Identities Datamart Environment (TIDE) after he applied in May to study at a bogus college. But there was apparently not enough information about Abdulmutallab's activities to move him to a watch list that would have prevented him from flying.
The names of more than half a million individuals are on the TIDE list.
Abdulmutallab, who received a degree in engineering and business finance from University College London in June 2008, applied in May 2009, to re-enter Britain to study at another institution. But he was refused entry because officials suspected the school was not genuine.
Johnson said Monday that U.S. authorities should have been informed that Abdulmutallab had been placed on their list. But he added that he believed all procedures had been followed correctly.
The use of bogus schools to secure student visas has been identified as a weakness in Britain's immigration system. In April, one of several suspected terrorists arrested in raids in northern England was found to have a visa issued with the help of a fake college, prompting opposition lawmakers to call for a crackdown.
Last week, Britain's immigration minister Phil Woolas boasted that the government had closed some 2,000 fraudulent schools.
Meanwhile, a court hearing to determine whether the U.S. government can take Abdulmutallab's DNA has been postponed. The federal court in Detroit says a hearing scheduled for Monday has been delayed until Jan. 8. No reason was given.
It's not known why the government wants Abdulmutallub's DNA. Defence lawyer Miriam Siefer was researching whether she had a legal basis to stop it.
Abdulmutallub is being held at the federal prison in Milan, Michigan.
The lone wolf theory?
A prominent security expert says that with this week's attempted attack, it is possible security officials are witnessing a new trend in terrorism: the "lone wolf" terrorist.
Security consultant and former CSIS Agent Michel Juneau-Katsuya says the thwarted attack on Northwest Airlines flight from Amsterdam to Detroit might signal a new era.
"We're seeing something new: an individual totally unrelated to al Qaeda who embraces the cause and ideology of al Qaeda and decides themselves to launch an attack," Juneau-Katsuya noted on Canada AM.
"Ultimately, if that is the case, that would be the worst-case scenario for investigators," he added.
It's relatively easy to investigate a group because groups leave bigger "footprints" as they communicate with one another as they plan attacks, Juneau-Katsuya noted. But a single individual can go under the radar for a long period of time, as he quietly plans an attack using information on building weaponry easily obtained from the Internet.
Juneau-Katsuya says a new era in terrorism might result in authorities taking a new approach to the so-called war on terror.
"We've been attacking it from a security point of view... but maybe it's time to go to the real source of terrorism: understanding the political, economic and legal justification and grievances that motivate terrorists to start attacking us," he said.
With reports from The Associated Press
