LONDON (Reuters) - Britain's security services could not have prevented two men hacking to death a British soldier in broad daylight on a London street last year, even though one of the killers had been investigated five times, lawmakers said on Tuesday.
However, parliament's Intelligence and Security Committee did say that one unspecified U.S. communications firm could have led British investigators to head off the plot.
Michael Adebolajo, 29, and Michael Adebowale, 23, both Muslim converts, ran over Fusilier Lee Rigby at speed near Woolwich Barracks in May 2013 before setting upon him with knives and a meat cleaver in an attempt to behead him.
They asked bystanders to film them with bloodied hands, calmly justifying their actions as a response to Britain's foreign policy. They were jailed for life in February.
The committee said in a report that, although mistakes had been made, Britain's three security agencies could not have prevented the attack.
"We do not consider that these errors taken individually were significant enough to have actually made a difference," committee chairman Malcolm Rifkind told reporters.
The committee noted that the domestic spy agency MI5 had investigated Adebolajo five times, twice as a high priority, but had found no evidence that any attack was being planned.
Adebowale had also told an overseas militant in an online exchange in December 2012 that he wanted to kill a soldier, but this only came to light after the murder.
The committee said the unnamed U.S. communications firm had picked up the exchange, but had not informed MI5. If it had, there was a "significant possibility" that the attack could have been stopped.
Rifkind said U.S. Internet companies such as Apple, Google, Facebook, Microsoft, Yahoo and Twitter needed to do more.
"Their networks are being used to plot murder and mayhem," Prime Minister David Cameron told parliament. "It is their social responsibility to act on this."
Cameron pledged another 130 million pounds ($200 million) over the next two years to enhance the monitoring of hard-to-detect "self-starting terrorists".
Adebolajo's brother Jeremiah said in a statement that the report would be used to justify additional surveillance and prevention powers announced on Monday by the government, further alienating young Muslims.
(1 US dollar = 0.6381 British pound)
Editing by Kevin Liffey