ABUJA - The family of a Nigerian man who tried to blow up a U.S. passenger airliner said on Monday they had lost contact with him while he was studying abroad and reported his disappearance to security agencies two months ago.

In the United States, authorities tightened airline security and questions were raised about how a man suspected of ties to militants was able to smuggle explosives aboard a transatlantic flight.

Umar Farouk Abdulmutallab, 23, was charged on Saturday in the United States with trying to blow up Northwest Airlines flight 253 as it approached Detroit from Amsterdam on Christmas Day with almost 300 people on board.

In a statement to Nigerian media, the Mutallab family said: His father, having become concerned about his disappearance and stoppage of communication while schooling abroad, reported the matter to Nigerian security agencies about two months ago and to some foreign security agencies about a month and a half ago.

It said the fact that he had ended communication with his family was completely out of character and a very recent development.

Nigerian media had quoted family members as saying the father had been uncomfortable with his son's extreme religious views and had reported him to the U.S. embassy.

Dutch military police said they were investigating the possibility that Abdulmutallab might have had help from an accomplice before boarding the flight at Amsterdam's Schiphol Airport.

A U.S. couple on the flight, Kurt and Lori Haskell, told Reuters and other news agencies they saw a tall, well-dressed man aged about 50 with Abdulmutallab on Friday morning.

At this moment we have no information on whether there was another guy, a military police spokesman said. We are checking all clues and information we get.

The U.S. Transportation Security Administration said it had stepped up pre-flight screening in the United States and Europe.

Air travellers also described new in-flight restrictions, including a ban on unescorted toilet access and on passengers carrying anything on their laps an hour before landing.

U.S. Homeland Security Secretary Janet Napolitano said the government was investigating whether al Qaeda was involved with Abdulmutallab, who was added last month to a data bank of names of purported terrorist sympathisers after his father's warning to the U.S. embassy.

A White House spokesman said President Barack Obama ordered a review of how suspects' names are added to counter-terrorism watch lists, suggesting procedures might be outdated.

I do think though that in many ways this system has worked, White House spokesman Robert Gibbs told CBS television. We just have to continue to keep refining it and stay ahead of what terrorists are trying to do.

Senate Republican Leader Mitch McConnell said on ABC television: There's much to investigate here. It's amazing to me that an individual like this who was sending out so many signals could end up getting on a plane going to the U.S.

CNN quoted an unidentified source with knowledge of the investigation as saying the suspect carried enough explosive material to blow a hole in the plane, had it been detonated.

A U.S. law enforcement official, speaking on condition of anonymity, confirmed reports that Abdulmutallab had told investigators al Qaeda operatives in Yemen had given him an explosive device and told him how to detonate it.
The son of a respected former banker, he is from a privileged background in Africa's most populous country, most of whose estimated 140 million people live on less than $2 a day.