The grim possibility that the two London attacks were not simply a sporadic terror campaign is being discussed at the highest levels in Whitehall, the centre of the British government.
Fears of a third strike remained high this weekend, based on concrete evidence supplied by an intercepted text message and the interrogation of a terror suspect being held outside Britain, say US reports.
As police and the security services work to prevent another cell murdering civilians, attention is focusing on the pool of migrants to Britain from the Horn of Africa and central Asia.
Intelligence service MI5 is working to an estimate that over 10,000 young men from these regions can use light weapons and military explosives.
A well-connected source said there were more than 100,000 people in Britain from "completely militarised" regions, including Somalia and its neighbours in the Horn of Africa, and Afghanistan and territories bordering the country.
A very high proportion were Muslim men of military age.
"Every one of them knows how to use an AK-47 [automatic rifle]," said the source.
"About 10 per cent can strip and reassemble such a weapon blindfolded, and probably a similar proportion have some knowledge of how to use military explosives. That adds up to tens of thousands of men."
Even though the vast majority had come to Britain to escape the lawlessness of their homelands, the source added, there remained an alarmingly large pool of trained men who could be lured into violent action here.
This threat had been largely neglected while attention focused on British-born militants who had been through training camps run by al Qaeda in Afghanistan.
"There has been a debate on whether we are facing an insurgency or terrorism," said the source, "and the verdict is on the side of an insurgency."
Against this background, as many as 400 more armed police may be recruited in London. Concern has been expressed in the wake of the massive anti-terror operations that the police are being overstretched.
Sir Ian Blair, the Metropolitan Police Commissioner, is to recruit hundreds more officers. He warned that his armed officers were suffering from fatigue after weeks of round-the-clock duty.
The Prime Minister, meanwhile, has ordered a government-wide drive to neutralise opposition within the Muslim community to his package of proposed anti-terror measures.
- INDEPENDENT