Thousands of British air passengers are facing lengthy delays amid security tensions in the wake of the botched Detroit airliner bomb plot.
As intelligence agencies probed the background of the former London student who allegedly tried to ignite a device on Christmas Day, there were anxious scenes at the US airport on Monday night when a second Nigerian man was arrested on the same route.
A security official at Detroit airport confirmed later he posed no security risk to the plane. He was a businessman who had become "verbally disruptive" after falling ill during the flight and locking himself in the bathroom for about an hour, the officer said.
In Britain, passengers waiting to fly to America for New Year were hit by delays of up to three and a half hours while urgent investigations took place to find out how Umar Farouk Abdulmutallab - who was known to US and British authorities - was able to take explosives on board a transatlantic airliner carrying 278 passengers.
Scotland Yard detectives continued to search the 23-year-old's luxury apartment in a central London mansion block as it emerged an al Qaida operative in Yemen threatened the US and said "we are carrying a bomb" in a video posted online four days before the attack. The former University College London (UCL) student also reportedly visited East London Mosque in Whitechapel, east London, which has been linked to Muslim extremism, on a number of occasions.
Security sources said that both the police and MI5 were diverting extra staff and resources to the investigation as a matter of priority in order to establish the significance of the "London link".
Abdulmutallab's family later released a statement saying he disappeared while studying abroad. It said they approached Nigerian security agencies after he stopped communication and then contacted foreign security agencies for "their assistance to find and return him home". The family said: "It was while we were waiting for the outcome of their investigation that we arose to the shocking news of that day."
And Home Secretary Alan Johnson confirmed that Abdulmutallab had been placed on a UK watch list after he was refused a student visa. "If you are on our watch list then you do not come into this country," Mr Johnson told BBC Radio 4's Today programme. "You can come through this country if you are in transit to another country but you cannot come into this country."
People flying across the Atlantic were advised to arrive early and carry just one piece of hand luggage. One flight from London's Heathrow to New York's Newark airport was late leaving by more than three hours, although most delays were less severe.
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Meanwhile, US President Barack Obama ordered a fresh review of screening processes. US homeland security secretary Janet Napolitano released a statement saying the additional security measures were "designed to be unpredictable" and passengers "should not expect to see the same thing everywhere".
