The US Federal Bureau of Investigation (FBI) said that preliminary analysis had shown that the suspect had strapped pentaerythritol, which he tried to ignite, to his body.

Passengers said that the suspect's trousers and the aircraft wall caught fire before he was overpowered by passengers and crew.

US media, citing anonymous officials, reported that Abdulmutallab had admitted to having explosive powder taped to his leg that he sought to ignite by using a syringe filled with chemicals.

Screams of fear

Richelle Keepman, a passenger aboard the plane, said she first noticed a disturbance in the cabin when she heard screams.

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"We were in the back of the plane and we heard some screams and some flight attendants ran up and down the aisles," she said.

"I think we knew at the point when we saw the fear in the flight attendants' eyes and they grabbed the fire extinguishers."

US officials described the incident as an "attempted act of terrorism".

The Delta Airlines Airbus, with 289 people on board, was on final approach to the US city of Detroit from Lagos, via Amsterdam, when passengers say they saw a puff of smoke and heard a sound like firecrackers.

British police raided several addresses in London, where Abdulmutallab was believed to have previously studied .

"We are in liaison with the US authorities and searches in London are being conducted as part of ongoing enquiries," the Reuters news agency quoted a spokeswoman for the Metropolitan Police as saying.

"The searches are being carried out at more than one address," she said, declining to give further details.

'Al-Qaeda links'

Al Jazeera's John Terrett, reporting from Washington, said that Abdulmutallab was reported to have claimed he was trained by al-Qaeda in Yemen.

Abdulmutallab was reported as saying he had been trained by al-Qaeda in Yemen [Reuters]
"[He] says he has links to al-Qaeda and he says that he got the equipment that he allegedly used as the flight was approaching Detroit from contacts in Yemen," Terrett said.

In Nigeria, the government confirmed the suspect was one of their citizens and said Nigerian authorities had launched an investigation.

Al Jazeera's Yvonne Ndege, reporting from Abuja, said that authorities would be concentrating on how he breached security and boarded the aircraft. Abdulmutallab is believed to have boarded the flight in Lagos.

Ndege saidd that Abdulmutallab's family would be questioned, but that there is no suggestion that they are involved in illegal activities.

One of his uncles said that Abdulmutallab was estrained from the family because of his extreme religious views.

Enhanced screening

In the US, homeland security officials said enhanced security measures had been put into effect after the failed attack.

"Passengers may notice additional screening measures put into place to ensure
the safety of the travelling public on domestic and international flights," they said in a statement.

Officials have said that that Abdulmutallab was added to the Terrorist Identities Datamart Environment (TIDE) security watch list, which includes about 550,000 people in November 2009.
  
However, the Nigerian was not on the smaller Terrorist Screening Data Base (TSDB) and was not flagged for mandatory secondary screening or included on the "no fly" list, an official told the AFP news agency.

Barack Obama, the US president who is currently on holiday in Hawaii, was "actively monitoring" the situation, a White House spokesman said.

"The president was notified of the incident this morning between 9am (07:00 GMT) and 9.30am Hawaii time by the president's military aide," Bill Burton said in a statement.

After Obama was informed of the incident he held a secure conference call with John Brennan, his homeland security and counterterrorism adviser, and Denis McDonough, the National Security Council chief of staff .

"[Obama] asked to arrange a subsequent secure call and... instructed that all appropriate measures be taken to increase security for air travel," the White House said.