A report examining what MI5 knew about the 7 July London bombers before their 2005 attacks is to be published later.
A parliamentary committee has cleared the security service MI5 of failings over the 7/7 London suicide attacks.
The parliamentary Intelligence and Security Committee report was ordered after a trial heard ringleader Mohammad Sidique Khan had been followed in 2004.
The Intelligence and Security Committee report reveals in unprecedented detail what officers knew of the 2005 plot's ringleader Mohammad Sidique Khan.
The fresh report is expected to detail the nature of MI5's surveillance of him and accomplice Shehzad Tanweer.
But it stresses that none of the information amounted to evidence that he was a threat to national security.
An earlier report cleared the security service because there had been no intelligence the men were a risk.
The committee said MI5 was stretched almost to breaking point in 2004, and officers had more urgent priorities.
The Intelligence and Security Committee (ISC), which reports to the prime minister, revealed in 2006 that Khan and Tanweer had appeared on the periphery of another investigation.
The committee did accept MI5's assessment that Khan knew other terrorists.
The report said neither man had been fully identified and there was no intelligence to suggest that they were plotting an attack.
But its report suggests that in 2004, a year before the attacks, MI5 was preoccupied as teams traced bomb plotters around the UK.
But in 2007, it was revealed that the men had in fact appeared on the security service's radar thanks to their contacts with another group of men later arrested for plotting fertiliser bomb attacks.
It reveals for the first time that during that year 52 suspects were classed as "essential targets", but MI5 did not have the manpower to watch them.
Officers saw Khan attend four meetings with some of those plotters during February and March 2004. At least one conversation was caught on an MI5 bug.
In that context, the committee said the security services' decision not to follow Khan after he had initially appeared on their radar was understandable, taking into account operational pressures.
On a separate occasion, officers followed Khan from Sussex to his home in Leeds.
Fifty-two people died and hundreds were injured when Khan and three other suicide bombers attacked the London transport system.
According to the original ISC report, officers decided not to investigate any further because of more urgent priorities.
The new report was ordered by Prime Minister Gordon Brown in the wake of the 2007 revelations - but its publication was delayed because of a prosecution relating to the London bombings.
Survivors of the 7/7 attacks and families of victims have been demanding to know why the security service decided to discount the Leeds pair.
Some families say they will push for an independent inquiry if the ISC report fails to answer all of their questions.
Fifty-two people died and hundreds were injured when Khan, Tanweer and two other suicide bombers attacked the London transport system.
