The murderers: Michael Adebolajo and Michael Adebowale
The big internet companies have a "social responsibility" to act on terrorist material posted online, Prime Minister David Cameron has said.
It comes after a report into Fusilier Lee Rigby's murder found one of his killers spoke online about murdering a soldier five months before the attack.
The intelligence agencies did not know about this exchange, understood to have been on Facebook, until afterwards.
Facebook said it did not allow terrorist content and aimed to stop it.
Human rights organisations accused the inquiry of shifting the blame on to internet companies and away from the intelligence agencies.
The report comes a day before the government outlines plans to increase powers for police and security services.
Fusilier Rigby was killed on 22 May last year, near Woolwich Barracks in London.
Michael Adebolajo and Michael Adebowale drove into the 25-year-old before hacking him to death.
Culled from BBC News.