BRITISH SHUT DOWN HEATHROW AS TERROR ATTACK THWARTED

British officials of MI-6 have taken twenty-one suspects into custody and have closed Heathrow Airport to inbound traffic as part of a sweep that stopped a massive terrorist attack. The UK went to its highest terror alert as they unraveled what appears to be a home-grown plot:

A plot to blow up planes in flight from the UK to the US and commit "mass murder on an unimaginable scale" has been disrupted, Scotland Yard has said. It is thought the plan was to detonate explosive devices smuggled in hand luggage on to as many as 10 aircraft.

Police were searching premises with 21 people in custody after arrests in the London area and West Midlands. ...

According to BBC sources the "principal characters" suspected of being involved in the plot were British-born. BBC home affairs correspondent Andy Tighe said police sources had told him they had found "interesting items" which were being examined.

He said police acted when "something happened".

The reports are still sketchy, but the operation looks to have come to MI-5's attention some time ago, and that they waited until it went active to pull the trigger. The British had coordinated with American security officials, and the US raised the threat level for British inbound flights to our highest level, the first time international flights have received that threat indicator. MI-5 believes that the terrorists targeted flights to the US.

Authorities in Britain have not taken any chances. Knowing that al-Qaeda has employed back-up schemes, they have imposed tight restrictions on travel. Since this plot involved terrorists taking explosives onto planes using carry-on luggage, they have essentially banned them from flights for the near future. The restrictions are so tight that passengers will not be allowed to take liquids, in case they try masking explosives as bottled drinks.

So far, the BBC is reporting that security officials consider this to involve British citizens, and not foreigners. However, the profile certainly suggests al-Qaeda involvement. AQ has specialized in hand-carried suicide bombs on transportation systems in the last few years; 9/11 was maybe the only major AQ operation that did not use that modus operandi, and that was because the terrorists used the planes themselves as the explosives. The BBC report mentions that British officials were communicating with a number of "community leaders" to keep them apprised of the situation, which sounds as if they may have been tipped by these people to the plot and/or the plotters.

This will be the story of the day. Be sure to check with Michelle Malkin, who will update this as it develops. Hot Air's Allahpundit will also keep up with the late-breaking details.

UPDATE: It Shines For All points to a disturbing discrepancy in the numbers coming from the AP's security sources:

Police arrested 21 people in London, its suburbs and in Birmingham as part of a major covert counterterrorism operation that had lasted several months, Deputy Commissioner Paul Stephenson said. Searches continued in a number of locations ...

A senior U.S. counterterrorism official said authorities believe dozens of people -- possibly as many as 50 -- were involved in the plot, which "had a footprint to al-Qaida back to it." The official spoke on condition of anonymity because of the sensitivity of the situation.

It means that less than half of the suspected plotters have been detained, at least by AP's report.

UPDATE II: Does this bring back any memories? How about Operation Bojinka, al-Qaeda's first attempt to exploit commercial aircraft for a coordinated terrorist attack?