"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 US," -- Senate Republican Leader Mitch McConnell.
The United States tightened airline security amid growing questions about how a Nigerian man with alleged ties to militants smuggled explosives aboard a transatlantic flight and attempted to blow it up.

The Transportation Security Administration said Sunday it had stepped up pre-flight screening in the United States and Europe. Flyers also described new in-flight restrictions, including a ban on unescorted restroom access and carrying anything on a passenger's lap an hour before landing.

The Obama administration was investigating whether al Qaeda was involved with Umar Farouk Abdulmutallab, the Nigerian charged with trying to bring down the Delta Air Lines flight from Amsterdam to Detroit on Christmas Day, said Homeland Security Secretary Janet Napolitano.

Abdulmutallab's name was added last month to a central US database, containing about 550,000 names of purported terrorist sympathizers, after his father warned the US embassy about his alleged radicalization.

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suspects' names are added to counter-terrorism watch lists, a White House spokesman said, suggesting procedures may be outdated.

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

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 US 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.

In Yemen, al Qaeda vowed to take revenge over raids against the group this month that it said were carried out by US warplanes.

Republicans appearing on Sunday television programs questioned whether the Obama administration was doing enough to contain security threats.

"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 US," Senate Republican Leader Mitch McConnell said on ABC.

"Radicalization is alive. It is well. They want to attack the United States," Representative Peter Hoekstra, ranking Republican on the House Intelligence Committee, told "Fox News Sunday." "I think this administration has downplayed it."

Abdulmutallab started his journey in Nigeria's commercial hub of Lagos, where he boarded a KLM flight to Amsterdam before going through another security checkpoint at Schiphol airport, Dutch counter-terrorism agency NCTb has said.