Police and Britain's MI5 spy agency likely missed chances to identify the key men behind the 2005 London bombings, but could have done little more to halt the attacks given an alarming lack of resources, lawmakers said Tuesday.

A detailed report into what law enforcement officials knew before the transit attacks that killed 52 people and the four bombers reveals how one of the assailants was tracked by intelligence officers as early as 2001.

The same suspect and another bomber were repeatedly watched in the company of other terrorists but never pursued, because officials were overwhelmed with other threats perceived at the time to be imminent and more serious.

"There will always be gaps in intelligence coverage," a lawmaker concluded in the report. "It is an uncomfortable truth that, at some time in the future, and without any prior warning, it is very possible that the U.K. will be the subject of another terrorist attack."

One conversation between London homicide bomber Shehzad Tanweer and another suspected terrorist wasn't fully transcribed for four years, the report said. Poor communications between MI5 and police may have meant some possible connections between suspects weren't made. In another instance, sketchy surveillance meant British intelligence confirmed only after the attacks that Tanweer and ringleader Mohammed Siddique Khan had met a Canadian bomb making expert in 2004.

Both men had been dismissed as petty fraudsters who were unlikely to be planning a bomb plot.

The inquiry, by Parliament's Intelligence and Security Committee, discloses that in 2004, MI5 had patchy, poor or no surveillance on 94 percent of suspected terrorists — including London bombing ringleader Khan and Tanweer.

Only 6 percent of suspects were adequately kept under surveillance, due to limited resources, the report said.

"These are astounding figures," lawmakers acknowledged. It was their second inquiry since 2006, necessary after new intelligence about the bombings came to light.

Jonathan Evans, the head of MI5, says there are around 2,000 known terrorists operating in Britain. Security officials say there are likely thousands more who are unknown to authorities.

Even now, MI5 only has the manpower to "hit the crocodiles nearest the boat," Evans told lawmakers in an evidence session held in private.

Lawmakers said that to properly cover the current threat, MI5 would need several hundred thousand officers — it currently has about 3,500.

Committee chairman Kim Howells said few Britons wanted a vast secret police with scores of officers and tens of thousands of informants.

Between an arrest in 1993 by police for assault and the time of the bombings, Khan was photographed, followed, or listened to on audio bugs by MI5 and police at least eight times. He was one of 40 men spotted at a 2001 British terrorism training camp, the report said.

Tanweer was monitored on seven occasions and arrested on a harassment charge in April 2004, the report said.

In February 2004 and March 2004, Khan and Tanweer were watched repeatedly meeting with Omar Khyam, a British man jailed for life in 2007 for leading a plot to bomb nightclubs and power plants.

But MI5 and police did not know who Khan and Tanweer were, and assumed they were interested in fraud or credit card scams. It was only after the 2005 bombings that officers discovered that Khan and probably Tanweer met with Canadian bomb expert Momin Khawaja for a meal on Feb. 21, 2004, lawmakers said.

With a limited number of officers, and a series of bomb plots to watch, the spy agency and police could not afford to stay on the trail of the two men. Khyam, who was MI5's key target, had around 4,000 telephone contacts, all of which needed to be checked. Those links exposed four terrorist plots, all of which were prioritized over Khan.

Legislators said no blame should be directed toward MI5 or police, who have halted 12 major plots against Britain since 2001. "We cannot criticize the judgments made by MI5 and police based on the information that they had, and their priorities at the time," the report states.

Some victims and bereaved families said that lawmakers had failed to hold police and MI5 accountable.

"I think it's quite shameful," said Graham Foulkes, whose 22-year-old son, David, was killed in the London attacks.

Foulkes and others, including some lawmakers, have called for a judge-led inquiry to decide whether or not MI5 or police made mistakes that could have halted the bombings.

In a rare statement, MI5 said it had been vindicated by the inquiry. "There is no evidence to support claims of overlooked clues and ignored warnings," the agency said on its Web site.