MI5 Spared Major Criticism Over Rigby Killing

The murder of soldier Lee Rigby was random and largely unpreventable, a major year-long investigation is set to reveal.

A report by Parliament's Intelligence and Security Committee is expected to largely clear MI5 of serious failings but highlight some areas where the security service could have performed better.

The report will be published on Tuesday, when David Cameron will also address the House of Commons.

He had asked the committee to investigate after questions emerged in the days after the killing in southeast London on 22 May 2013.

The committee has examined what, if anything, MI5 knew about the two killers, Michael Adebolajo and Michael Adebowale.

Their report is said to be "substantial" - the product of written and oral evidence, and hundreds of pages of primary-source material.

It will confirm that both men were known to MI5, in particular 29-year-old Adebolajo.

He was arrested, with others, in Kenya in 2010, trying to cross into Somalia to fight for terrorist group al Shabaab.

The Kenyans passed him over to the British and MI5 tried to recruit him as an informant.

What happened then, will form a major section of the report.

MI5 has always maintained that although Adebolajo was on their radar, he was not considered a high priority, and therefore not worthy of a time-consuming, resource-sapping surveillance operation, which are reserved for the most dangerous suspects.

Sky News understands the investigation is likely to sympathise with that decision, although the committee, chaired by Sir Malcolm Rifkind, will say there were clues in the online behaviour of Adebolajo.

While it might have been possible to uncover his plot if his online activity been monitored, requests to monitor his phone or internet activity would not have passed the requisite threshold because he was not deemed high risk.

This will prompt the ISC to strongly recommend that working relationships between the security services and internet companies need to improve.

It will also warn MI5 that it must be more rigorous in its record keeping, at a time when the country faces a threat from potential terrorists returning from Iraq and Syria.