The homicide bombers who killed 52 passengers on London's transit system last year contacted someone in Pakistan just before striking, Britain's top law enforcement official said Thursday.

Home Secretary John Reid also told the House of Commons that police and intelligence agencies had thwarted three attacks since the July 7 bombings, but did not say who was behind them or where and when they were to take place.

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Reid's statement indicated that a third bombing attempt had been stopped since February when Peter Clarke, deputy assistant commissioner of the Metropolitan Police, said two attacks had been thwarted since the transit bombings.

A report by the Intelligence and Security Committee concluded that intelligence agents had been alerted to two of the homicide bombers before the attacks but limited resources prevented them from uncovering the plot.

The committee said intelligence agents did not concentrate on the two terror suspects — Siddique Khan and Shazad Tanweer — because they were not believed to be an urgent threat and investigators decided to focus on "known plans to attack the U.K."

Reid, speaking of the contacts ahead of the attacks, said authorities did not know what was discussed. Authorities have long held that the four bombers were homegrown terrorists who acted alone.

"There are a series of suspicious contacts from an unknown individual or individuals in Pakistan in the immediate run-up to the bombings," Reid said after his department released its narrative of the attacks. "We do not know their content."

Three of the bombers' families were originally from Pakistan and several had traveled there before the attacks.

The report also said the degree of Al Qaeda involvement in the attacks, if any, was unclear, and it found no links between the attacks and the people who mounted failed bombing attempts against the London transport system two weeks later.

The four bombers killed themselves and 52 others when they detonated backpacks jammed with explosives on three subway cars and a bus — the worst terrorist attack in British history.

The investigators said this decision was understandable given the scale of the threat and limited resources, concluding that British intelligence agencies had "no culpable failures."

Committee chairman Paul Murphy said there was no warning before the attacks, but more intelligence resources in Pakistan and at home might have helped authorities detect the plot.

"Neither the potential speed of radicalization nor the fact that British citizens could be radicalized to the point of suicide were understood" by security services before the attacks, Murphy said, adding the committee was concerned "this could have had an impact on the ability of authorities to respond."

"Greater coverage in Pakistan or more resources generally in the U.K. might have alerted the agencies to the intentions of the 7th of July group," he added.

The Intelligence and Security Committee — a panel of lawmakers that meets in secret to scrutinize intelligence work — interviewed the heads of Britain's two spy agencies as part of its work.

The report said intelligence agents were previously aware of Khan, leader of the attacks, but had not confirmed his identity until after the attacks.

The decision was made not to investigate Khan and Tanweer because "there were more pressing priorities at the time, including the need to disrupt known plans to attack the U.K," the report said.

Murphy said no evidence at the time indicated that the two men were involved in any attack plan against the country.

"When resources became available, attempts were made to find out more about these two and other peripheral contacts, but these resources were soon diverted back to what were considered to be higher investigative priorities," it said.

The man later identified as Khan had been mentioned in reports from people detained outside Britain in 2004, the report said. "This reporting referred to men from the U.K. known only by pseudonyms who had traveled to Pakistan in 2003 and sought meetings with Al Qaeda figures," the committee said.

The report also confirmed that Britain's alert status had been downgraded from "severe general," the second-highest, to "substantial" on May 26.

But Murphy said the committee believed it was unlikely that the lowering of the threat rating had hurt the chances of preventing the attacks or lowered responders' alertness.

The Home Office report has been widely seen as an alternative to a full public inquiry, which could have heard evidence in open sessions.

Survivors of the bombings are campaigning for a public inquiry similar to the U.S. commission into the Sept. 11, 2001 attacks in the United States, claiming the reports are not likely to address any mistakes made by government officials.