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'Mass murder on an unimaginable scale'

Confusion reigned at Heathrow International Airport, even as security measures eased a bit with the lowering of the threat level. NBC's Lisa Myers reports.

The threat level here was lowered Monday from critical to severe, but the country remains on edge.

Confusion reigned at Heathrow International Airport, even as security measures eased a bit with the lowering of the threat level.

This means that the terrorist attack is still highly likely, but the intelligence assessment suggests that such an attack is no longer imminent.

Intelligence sources say the investigation has turned up new evidence that has changed some thinking about the plot but not the overall assessment.

"Put simply, this was intended to be mass murder on an unimaginable scale," Paul Stephenson, Deputy Commissioner, (London) Metropolitan Police.

Here's what we know now:

The plot was to blow up nine planes bound for the United States as they crossed the Atlantic. Actual flights and dates had not yet been chosen.

A British official tells NBC News it's now believed there was to be only one suicide bomber per plane, instead of two as previously believed.

Authorities call the bomb’s design ingenious — made of commonly available chemicals and components which could easily slip through security.

Authorities intercepted a message that the design had been tested in Pakistan and worked.

"This could mean that the plot was about to enter the execution phase," says Roger Cressey, NBC News terror analyst.

Twenty-three alleged plotters are under arrest in Britain and at least seven in Pakistan.

The alleged ringleader, Rachid Rauf, is believed to be about 25, trained in explosives and under arrest in Pakistan. Authorities tell NBC that Rauf was already wanted for murdering a relative in Britain.

Those arrested in London range in age from 17 to 35. Most are British citizens of Pakistani descent. Two are women. Some are recent converts to Islam.

A Western counter-terrorism official describes most of them as "apprentices," though a few were known to British intelligence. Several had made martyrdom videos.

"It's a combination of a local cell with an international link that makes it very seriously dangerous," says Mike Sheehan, NBC News terror analyst.

The Pakistan connection is strong. Some British suspects trained in camps there and money was wired from Pakistan to pay for plane tickets.

British and American officials tell NBC there was a significant dispute over when to roll up this plot. The British wanted to wait at least another week until the plotters moved toward executing a dry run, but the United States insisted on shutting down the operation now.

"I know the U.S. government was very keen to move forward, very keen to be able to make public the concerns that it had so that security levels could be raised," says Lord Toby Harris, Metropolitan Police Authority Oversight Board.

Al-Qaida's role in all this remains murky, but many officials do see fingerprints.

The plot itself was right out of al-Qaida's playbook, strikingly similar to one previously foiled in the Philippines to blow up airliners over the Pacific.

"Did they have operational control? Probably not. Were they aware of the operation? I think that's very likely," says Cressey.

President Bush said today this looks like the kind of thing al-Qaida would do, but so far there's no proof.