London ? European anti-terror officials helping their British counterparts investigate last week’s bombings here expressed increasing concern Monday about the apparent lack of progress in the case and the potential for new attacks.

British officials urged patience, saying they were focused on painstaking forensics work. The hunt for clues has been slowed by difficult access to subway tunnels and the size of the scene of a bus blast, British and European investigators said. As the work continued, the death toll rose to 52, with 700 injured.

A British law enforcement official acknowledged that the nation’s spy services were caught off-guard by the plotters’ ability to avoid generating the warning signs that often precede attacks, even vague intelligence indicators, known as “chatter,” that can be understood only in retrospect.

“They had no idea it was coming,” said the British law enforcement official, who asked not to be identified. “There was no background noise, no last-minute intelligence.”

Speaking in Parliament on Monday, Prime Minister Tony Blair said: “I know of no intelligence specific enough to have allowed them to prevent last Thursday’s attacks. By their very nature, people callous enough to kill completely innocent civilians in this way are hard to stop.”

There are increasing fears among European anti-terror chiefs that the bombers were as adept at covering their tracks as they were at concealing their preparations.

“The British seem to have very little,” said a senior European police official who hurried to London this weekend to work with British counterparts. “I think part of the problem is that too many Islamic groups developed in Britain. You can monitor two or three, OK, but when you have 10, the situation becomes toxic. They were overwhelmed.”

The official and other colleagues from the mainland said they respect the professionalism of British investigators, who invited experts from an array of Western security services to a weekend briefing.

Nonetheless, European officials said the British provided little more information than they have in press briefings. Moreover, the British have made few requests for help tracking potential suspects, sharing evidence or pursuing leads, European and U.S. anti-terror officials said. That’s unusual in Europe, where Islamic networks inevitably sprawl across often-unguarded borders and where international cooperation has become routine.

Because British law enforcement has a meticulous and discreet style, it could be that the silence masks progress as authorities conduct delicate surveillance or nail down leads.

“Either they have nothing, or they have something good and have decided to keep it quiet for the moment,” a senior Italian police official said. “I hope for the sake of all of us that they have something good.”