A parliamentary report into the murder by Muslim extremists of a British soldier in London last year pointed Tuesday to intelligence failures but said the security services could not have prevented the attack.
Michael Adebolajo, 29, and Michael Adebowale, 23, were featured in a total of seven investigations by Britain's spy agencies in the years before they ran over and stabbed to death Lee Rigby outside his barracks in May 2013.
But an 18-month inquiry by parliament's intelligence and security committee (ISC) said: "We do not consider that, given what the agencies knew at the time, they were in a position to prevent the murder of Fusilier Lee Rigby."
It catalogued a litany of errors, from the failure of the MI6 foreign spy agency to follow up on Adebolajo's arrest on suspicion of terrorism offences in Kenya to the "unacceptable" length of time it took to take action in relation to Adebowale, despite his low priority status.
But it blamed an unnamed Internet company for failing to identify one "highly significant" piece of information -- an online exchange just five months before the murder in which Adebowale explained his intent to murder a soldier.
The December 2012 exchange with an individual known only as Foxtrot -- thought to be an extremist with links to Al-Qaeda in the Arabian Peninsula (AQAP) -- only came to light after the attack.
The ISC report criticised the fact that the company which hosted the exchange felt no obligation to monitor or report such potential threats.
"We find this unacceptable: however unintentionally, they are providing a safe haven for terrorists," said the committee chairman, lawmaker Malcolm Rifkind.
"The problem is acute: until it is resolved the British public are exposed to a higher level of threat."
The criticism will boost controversial government efforts to give the security services more power to monitor Internet communications.
Elsewhere, the ISC criticised the government's counter-terrorism programmes to prevent radicalisation were "not working", something of urgent concern given the hundreds of Britons heading to fight in Iraq and Syria.
In February, Adebolajo was ordered to spend the rest of his life in jail without the possibility of parole, while Adebowale was sentenced to a minimum of 45 years.