Pictures have been released in America of the burnt underwear worn by the man who allegedly tried to blow up a plane above Detroit on Christmas Day.
Meanwhile, an al-Qaida group claimed responsibility for the failed Detroit airliner plot as US President Barack Obama pledged to hunt down those responsible.
The photos appear to show explosives sewn into a pair of pants.
Investigators removed a 15 centimetre long packet of the explosive substance, PETN for examination.
Last night President Obama broke off his Christmas holiday to vow to track down anyone involved in the plot.
Mr Obama said American forces would “not rest” until the people behind the attempted “act of terrorism” were found.
Al-Qaida in the Arabian Peninsula said yesterday the action was retaliation for a US operation against the group in Yemen.
Umar Farouk Abdulmutallab, who is charged with attempting to destroy the Northwest Airlines Flight 253, was described as “well mannered, quietly spoken, polite and able” by his former tutors at the University College London (UCL).
A statement from the university’s Department of Mechanical Engineering, where Abdulmutallab studied for an engineering degree from 2005 to 2008, said: “During his time on the course, Mr Abdulmutallab never gave his tutors any cause for concern and was a well mannered, quietly spoken, polite and able young man.
“We are deeply shocked by the recent news concerning Mr Abdulmutallab.”
A UCL spokesman also said Abdulmutallab was president of the Islamic society at the institution between 2006 and 2007.
According to US network ABC News, Abdulmutallab told FBI agents there were more people “just like him” in Yemen ready to carry out attacks in the near future.
In his first public comments on the incident, Mr Obama said yesterday: “A full investigation has been launched into this attempted act of terrorism and we will not rest until we have found all those responsible.”
He added: “We will continue to use every element of our national power to disrupt, to dismantle and defeat the violent extremists who threaten us, whether they are from Afghanistan or Pakistan, Yemen or Somalia, or anywhere where they are plotting attacks against the US homeland.”
Speaking during a family holiday to Hawaii, he said he had ordered a review into airport security and the monitoring of suspected terrorists.
Abdulmutallab, who is being held at a prison in Michigan, allegedly tried to ignite a device as the Northwest Airlines flight from Amsterdam – carrying nearly 280 passengers – entered its final descent to Detroit on Friday.
Home Secretary Alan Johnson said it was unlikely Abdulmutallab would have been acting alone.
He also revealed the 23-year-old had been banned from entering Britain and was placed on a “watch list” earlier this year.
Speaking on BBC Radio 4’s Today programme, Mr Johnson said Abdulmutallab had been refused a new visa and monitored since last May after applying for a bogus course.
The issues being investigated by police and security services in this country included “what happened when he was in this country, was he radicalised in this country, was there any association with whoever may have been behind this plot”, according to Mr Johnson.
“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,” he added.
Abdulmutallab’s wealthy family said they believe he was radicalised while attending the British International School in Lome, the capital of Togo.
After he broke off contact, they approached foreign security agencies expressing concern about his state of mind and requesting “assistance to find and return him home”.
British police and MI5 have been diverting resources to probe the importance of the Nigerian’s London links.
Throughout the weekend, search teams combed the imposing mansion block in Mansfield Road, close to Oxford Street, where Abdulmutallab lived in the capital.
Al-Qaida in the Arabian Peninsula, in a statement posted on the internet, said the alleged attack exposed the “large myth” of American and international security services.
The group, which also issued a photograph of Abdulmutallab, said he had successfully penetrated all checkpoints, highlighting how “fragile” they were.
It said only a “technical error” had prevented the device from detonating.