That's according to the Times of London, which reported Sunday that the special units are tracking as many as 12 suspects with ties to al Qaeda. The paper said Scotland Yard is worried those people could be planning more suicide attacks, but there isn't enough evidence to justify arresting, or even detaining them.
That may have been the case a year ago, reports CBS News Correspondent Steve Holt, when Britain's domestic spy agency MI5 checked out Mohammad Sidique Khan, one of the July 7 bombers. At the time, investigators decided he wasn't a threat to national security, and stopped tracking him.
A senior government minister insists Britain did not have a permissive attitude toward extremist Muslim refugees before the London bombings, dismissing criticism that lax policies have made Britain a fertile recruiting ground for Islamic extremists.
In other developments:
Meanwhile, officers in the northern city of Leeds - a focus of the investigation so far - continued searching an Islamic shop and a house near the home of one of the four alleged bombers, 22-year-old Shahzad Tanweer.
MI5 began evaluating Khan during an inquiry that focused on an alleged plot to explode a large truck bomb outside a target in London thought to be a nightclub in Soho, the newspaper said. The private inquiry reportedly evaluated hundreds of potential suspects.
The Metropolitan Police declined to comment on the report, and a spokesman for Prime Minister Tony Blair's Downing Street office also refused to comment.
The July 7 bombings which killed 55 people on three underground trains and a double-decker bus have prompted the government to propose new legislation outlawing "indirect incitement" of terrorism, including praising those who carry out attacks.
Charles Falconer, the Secretary for Constitutional Affairs and Lord Chancellor, on Sunday rejected a suggestion that the government had previously been lax in its policies toward political refugees from Muslim countries and helped make Britain a fertile recruiting ground for Islamic terrorism.
"In terms of asylum, our policy is: If you are in fear of persecution, you are entitled to come here," the minister responded, speaking on British Broadcasting Corp. television. "Obviously, if you then seek to attack the very state that you come to, that gives rise to different questions.