The security service's limited resources meant extra checks were not carried out on the 7 July ringleader before the 2005 attacks, a report has said.
The Intelligence and Security Committee (ISC) describes in detail what officers knew of Mohammed Sidique Khan before the suicide bombings in London, which killed 52 people.
This latest report, the second by Parliament's ISC into the bombings, will not end calls for a full public inquiry.
But the 108-page report does reveal, in unprecedented detail, the workings of the security service MI5 and how it goes about the shadowy business of uncovering plots to kill Britons.
In compiling their report, the ISC members, who have high-level security clearance, have been given full access to police and MI5 officers and files.
They also have access to the heads of all three intelligence agencies: MI5, SIS (MI6), and GCHQ, the government's communications centre in Cheltenham.
'Astounding figures'
The whole reason for commissioning this second report is that it turned out that the bombers were not after all "clean skins", which means unknown to the police and MI5.
That is what the public were told in 2005, but in fact Khan and Shehzad Tanweer were both tracked on the fringes of another terror plot investigation.
So the ISC has looked at the question of prioritisation in surveillance.
Specifically, it examined whether MI5 should have made Khan a priority back in 2004 and whether this would have prevented the London bombings.
The key suspect in the plot which MI5 was tracking at the time, Omar Khyam (since jailed for life), had over 4,000 telephone contacts.
These are illustrated in a diagram of mind-boggling complexity on page nine of the report.
The ISC says that at the time when he and his associates were being watched, in 2004, MI5 could only provide good intelligence coverage for one of the suspects, with "less good" coverage of another.
The names were left blank along with the exact numbers MI5 could watch at one time because the ISC does not want to give potential attackers an advantage.
But the ISC does say that MI5 "could only provide a reasonable level of coverage for 6% of the overall known threat".
'Poor communications'
More than 60% of MI5 targets had coverage described as "inadequate" or "none" - among these were 52 "essential" targets that had no coverage at all.
The report said these were "astounding figures".
It also said that between July 2004 and July 2005, 130,000 man-hours of surveillance were dedicated to international counter-terrorism targets, supporting around 70 operations.
"These numbers demonstrate just some of the vast amount of intelligence that MI5 gathered and assessed in this period", the report states.
The report talks in detail about the role of the Executive Liaison Groups (ELGs) which are "unique to major covert terrorism investigations".
These allow MI5 - which we learn does not use faxes - to share secret intelligence with the police.
On the basis of this, decisions are then made on how to go forward and gather evidence on suspects for prosecution.
The report says MI5 retains the lead for collecting, assessing and exploiting intelligence while the police leads on gathering evidence, obtaining arrests and preventing risks to the public.
'Psychology of extremism'
The ELG, we are told, decides when to hand over tactical direction from MI5 to the police and when to take "executive action" to stop the suspected plot and arrest the suspects.
The ISC does not rule out that future attackers could still slip through the intelligence net at some stage.
But they do point out that the (sometimes poor) communications between the police and MI5 back in 2004 are now a thing of the past.
SURVEILLANCE MI5 GAVE TO TARGETS IN 2004
# Good coverage: 0.13%
# Less good (some gaps): 6%
# Significant gaps: 33%
# Inadequate: 42%
# No coverage: 19% Source: ISC
Previously, the police and MI5 had a sometimes fractious relationship bordering on rivalry, with officers reluctant to share information.
Now, with nine regional Counter Terrorism Units (CTUs) set up around Britain, the two organisations work side by side.
MI5, whose budget trebled between 2001 and 2008, has also devoted more resources to understanding the psychology of extremism to try to stay one step ahead.
TVNL Comment: This is a carbon copy of the 9/11 event. "Known" operatives conducting activities that mirror the drills that were taking place. They were patsies.But the fundamental dilemmas over priorities remain.
In meetings at Thames House, MI5's HQ in London, intelligence officers have to weigh up which resources to devote to which investigations.
Those decisions are often based on scant information, tip-offs or intercepts.
'Largely exonerated'
Many will be asking the key question - could it happen again? The depressing answer is yes, it could.
Intelligence, says one insider, is an incomplete business.
The ISC's findings have been greeted with relief tinged with sober caution.
Any conclusion holding the security service culpable for the bombings would have been hugely damaging and demoralising.
Instead, the ISC has largely exonerated MI5, as it did the first time round in 2006, but this time after viewing far more information.
The ISC's criticisms - of failures to sufficiently share information with the police and of inadequate record-keeping - are not considered serious enough to have made a difference.
They also refer to a time five years ago, before the national merger of MI5 and Police Special Branch efforts to combat terrorism.
But that is still unlikely to satisfy some who remain convinced that key information is being suppressed that would show that the security service could and should have prevented the London bombings.
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