MI5 officers assigned to investigate the lead bomber in the 7 July attacks were diverted to another anti-terrorist operation, the BBC has learned.
Bombs on three Tube trains and a bus killed 52 people
Mohammad Sidique Khan was known to the police for suspected petty fraud.
But sources have now told BBC News the security services had been so concerned about him they had planned to put him under a higher level of investigation.
A parliamentary report on the 7 July suicide bombings said the security services cannot be blamed.
The cross-party intelligence and security committee did not accuse any agency of negligence over the attacks.
But the cross-party committee is asking why the lead bomber, who was known to police, was not fully investigated, despite being known to security officials.
Four suicide bombs on three Tube trains and a bus killed 52 people and injured hundreds on 7 July 2005.
BBC security correspondent Frank Gardner said the committee had for six months been "interviewing and examining the work of the intelligence and security agencies to see if the July 7 bombings could have been prevented".
"Could they have been prevented with better intelligence? Yes. Could they have been prevented given the resources that the agencies had? They think probably not.
"They are not pointing the finger of blame at anybody," he told BBC Radio 4's Today programme.
"Inevitably there will be suspicions of a whitewash here, public suspicions."
But he added: "It certainly doesn't exonerate [the agencies] - the bombers got through, they failed to stop it happening.
"But was anybody actually negligent? No."
The committee did make a number of recommendations, including suggesting changes to the "secretive and complicated" system of alert and threat levels, Mr Gardner said.
The national threat level was lowered from "severe, general" to "substantial" just before 7 July 2005.
Committee members believe this made no difference to the bombers' plans, but that the public needs to be better informed.
The committee interviewed many members of the police and intelligence community.
Whitehall officials told the BBC that the risk of home-grown terrorist attacks on Britain has increased substantially since 2003.
They say that new plots are being detected, and that 50% of them involve British citizens living in the UK.
The committee said it accepted that gathering intelligence on the activities of British militants in Pakistan was extremely difficult prior to 7 July, but says it should still have been better.
Our correspondent said there had been a "sea change", with the Pakistani authorities becoming more co-operative, since the bombings.
He also said the committee was "very aware that the security and intelligence agencies - MI5 and MI6 - didn't have and still don't have enough people with languages - that's their problem".
"The whole 'oil tanker' of intelligence needed to turn round a lot faster than it did, away from the Cold War to confronting the threat of religiously-inspired terrorism."
Professor Anthony Glees, head of the Centre for Intelligence and Security Studies at Brunel University in west London, told BBC News the security service had failed.
"It is the body that is charged with having good predictive intelligence and what the London attacks show is that there was no good predictive intelligence," he said.
"They did not look carefully enough at the sort of people who might be tempted into becoming terrorists."