Civil liberties campaigners also questioned the timing of the release of the report just one day before the government's presentation to parliament on Wednesday of tough new counterterrorism measures to tackle the perceived heightened threat posed by Britons fighting in Syria.
"This is carefully choreographed. You've got the security apparatus investigating the security apparatus and deciding they need more money and more power so they can roll back civil liberties even further," Cerie Bullivant, a spokesman for CAGE, a human rights group, told Al Jazeera.
Tuesday's report by the UK parliament's Intelligence and Security Committee, which oversees the security services, largely exonerated them of blame over the killing of Lee Rigby, a British soldier who was hacked to death by two men near his barracks in Woolwich, south London, in May 2013.
Rigby's killers - Michael Adebolajo and Michael Adebowale - were both known to the intelligence agencies as individuals involved in Islamic extremism, and the report said that mistakes in monitoring them had been made. But it concluded the agencies had not been in a position to prevent the attack.
Facebook 'safe haven'?
Instead, it pointed the finger at an unnamed US-based internet company - subsequently identified in the media as Facebook - which it said had failed to report an online exchange in December 2012, in which Adebowale had expressed an intent to murder a soldier, and called for new powers to allow security agencies to monitor online communications.
"What is clear is that the one party which could have made a difference was the company on whose system the exchange took place," said Malcolm Rifkind, the committee's chairman. "This company does not regard themselves as under any obligation to ensure that they identify such threats... We find this unacceptable: however unintentionally, they are providing a safe haven for terrorists."
But critics said the report failed to adequately investigate allegations made by Adebolajo that he had been tortured in Kenya in 2010 after being detained there on suspicion of seeking to travel to Somalia to join al-Shabab, and that he had been subsequently harassed and threatened by MI5 on his return home in an effort to turn him into an informer.
Friends and family say Adebolajo believed he was tortured at the behest of British intelligence, and was asked questions he said could only have been fed to his interrogators by British sources. They say his experience in Kenya and the pressure exerted on him by MI5 influenced his subsequent actions and hardened his attitude.
Addressing camera phones at the scene of the murder, Adebolajo said the attack had been carried out in retaliation for British foreign policy, and to avenge Muslims killed by British soldiers.
"They began to harass him. They kept on calling him, they kept on asking him to meet, they kept on going to his house, and really disrupting his life," Jeremiah Adebolajo, Michael's younger brother, told Al Jazeera at the time of his conviction. "They were putting a lot of pressure on him to cooperate with them in one way or another."
'Ceased to exist'
Adebolajo's close friend Abu Nusaybah made similar allegations in an interview with the BBC at the time of the attack. He was arrested as he left the TV studio and subsequently jailed for three years on charges related to extremist material posted online.
To pass the blame to internet companies is to use Fusilier Rigby's murder to make cheap political points. Internet companies cannot and must not become an arm of the surveillance state.
But Tuesday's report mostly ducked those issues. It said it had found no evidence of harassment, and said the agencies had a policy of neither confirming nor denying whether people had been approached to be informants.
On the question of Adebolajo's allegations of torture, it acknowledged they had not been investigated thoroughly enough when he raised them in an interview with counterterrorism police on his return to the UK.
And it conceded the possibility of security service involvement in Adebolajo's case could not be ruled out because of a "close working relationship" between the UK government and the Kenyan counterterrorism unit that interrogated him.
Mohammed Akunjee, a lawyer representing Nusaybah, told Al Jazeera that the committee's failure to seek information from Adebolajo, who was jailed for life, and those who knew him concerning his allegations was bizarre.
"It misses the absolute core issues," said Akunjee. "This report seems to talk about all issues except for the most important one, which is what were the motivational forces at play in Michael Adebolajo's mind?
"MI5 spent a huge amount of resources trying to turn him and speak to him before he did what he did. The idea that they don't speak to him now is completely ludicrous if they want to avoid this sort of thing happening in the future. It's Frankenstein - they created him by making his life so difficult."
|Protesters holding placards stand outside the Old Bailey courts [EPA]
Sweeping security powers
The publication of the security report preceded the publication on Wednesday of the government's latest counterterrorism bill, which will give authorities new powers to confiscate passports, prevent Britons suspected of terrorism from returning to the country, and introduce a raft of new laws aimed at tackling radicalisation.
David Cameron, the British prime minister, also said the government would increase spending on the security services by £130 million ($204m) over the next two years.
In a statement to Al Jazeera, Jeremiah Adebolajo said: "We are seeing an increase in the powers of the security services, ostensibly for the safety of [British] citizens. But this report is nothing short of a distraction from the true root of the problem: the United Kingdom's continuing aggression against the faith and the people of Islam."
The conclusions of the report were also fiercely criticised by internet privacy campaigners.
"The government should not use the appalling murder of Fusilier Rigby as an excuse to justify the further surveillance and monitoring of the entire UK population," said Jim Kullock, the executive director of the Open Rights Group.
"To pass the blame to internet companies is to use Fusilier Rigby's murder to make cheap political points. Internet companies cannot and must not become an arm of the surveillance state."