Forensic officers have been searching an apartment block in central London
Police have been searching a number of properties in central London after a man was charged with trying to blow up a US airliner flying to Detroit.
Umar Farouk Abdulmutallab, 23, from Nigeria, is alleged to have had an explosive device hidden in his clothes.
Security has been stepped up at UK airports for flights to the US following the incident.
It has also emerged that the suspect, who previously studied in London, was denied a UK visa this summer.
Mr Abdulmutallab is said to have been an engineering student at University College London between 2005 and 2008.
But according to Whitehall sources he was refused permission to return by the UK Border Agency after attempting to apply for a course at a bogus college.
The Metropolitan Police have been searching a flat at an apartment block in Mansfield Street, Marylebone, and other properties in the capital.
Meanwhile, in the US a federal judge has formally charged Mr Abdulmutallab with attempting to destroy a Northwest Airlines aircraft on Christmas Day.
The hearing took place in Mr Abdulmutallab's hospital room at the University of Michigan Medical Center where he is being treated for burns.
An affidavit filed in support of the charge stated: "As the flight was approaching Detroit Metropolitan Airport, Abdulmutallab set off the device, which resulted in a fire and what appears to have been an explosion."
Passengers on Flight 253 say a man was overpowered after trying to ignite an explosive device as the Airbus A330 came into land.
BBC security correspondent Gordon Corera said the device contained about 80g of a high explosive, which had been moulded around the suspect's body and was apparently able to pass undetected through airport checks.
It has emerged that Mr Abdulmutallab's father, a prominent Nigerian banker, flagged up to US authorities concerns about his son's extreme views.
He may also have been on the radar of UK intelligence but did not cause American officials serious concern and so was not on the US no-fly list.
It is understood one of the British authorities' key priorities will be to check whether he has cropped up in the course of any other investigations.
Transport Secretary Lord Adonis told the BBC that security had been tightened on transatlantic flights.
"We've agreed with the US authorities enhanced search regimes for passengers on flights to and from the United States," he said.
"Those have taken place immediately. They will lead to delays in flights to the US, which I'm sure passengers will understand in the circumstances."
UK airport operator BAA confirmed that passengers taking flights to the US would face increased searches before boarding.
The plane was carrying 278 passengers
The British Airways website, meanwhile, said that under revised security arrangements, travellers to the US would only be allowed one piece of hand luggage.
A spokesman said: "We apologise to passengers for any delays to their journeys. Safety and security are our top priorities and will not be compromised."
Simon Calder, travel editor of the Independent, told the BBC the extra checks were causing massive delays at a time when 25,000 people were leaving the UK on 100 daily US-bound flights.
"There's going to be more strain on the check-in desks, lots of people turning up at the airports don't know the new rule and of course there's lots of repacking," he said.
Mr Calder said questioning on US visa applications were unlikely to get more intrusive because it appeared Mr Abdulmutallab had travelled despite being of interest to the authorities.
However, he said if the "pat-down" searches being imposed on travellers to the US were extended to all flights, it would get "expensive and very slow in terms of processing people".