The two men suspected of butchering a soldier on a London street in a gruesome Islamist attack were known to security agents, government sources suggested on Thursday.
Prime Minister David Cameron hinted that intelligence agencies would face probes into how the attackers slipped under the radar, but said police and security services "will not rest" until they bring those responsible to justice.
In a brazen attack in broad daylight on Wednesday, the pair hacked 25-year-old soldier Lee Rigby to death with knives including a meat cleaver near an army barracks in Woolwich, southeast London, before delivering an Islamist tirade to passers-by.
The suspects, believed to be Britons of Nigerian descent, were shot by police after the attack and spent the night in separate hospitals under armed guard.
Government sources said reports that the men were previously known to security agents "were not inaccurate".
"We will be looking into the possibility that they were known," a source said.
Hardline Islamist preacher Anjem Choudary told AFP he knew one of the suspects, who he referred to as "Mujahid" but media are naming as Michael Adebolajo, a 28-year-old Londoner.
"He used to attend some of our activities over the years. Very peaceful chap actually, not violent at all," he said, adding that he lost contact three years ago.
Chilling amateur footage showed the man thought to be Adebolajo with bloody hands, still holding the knives as he told a member of the public they killed the victim "because Muslims are dying daily by British soldiers".
Cameron cut short a visit to Paris to chair a meeting Thursday of the government's emergency response committee, COBRA, which had already met without him in the hours following the attack.
Speaking to reporters outside his Downing Street office afterwards, the premier branded the attackers "sickening" and described the killing as a "betrayal of Islam".
"This was not just an attack on Britain and on the British way of life. It was also a betrayal of Islam and of the Muslim communities who give so much to our country," he said.
He declined to comment on whether the men had been under surveillance, but said there may be a review of the work of the police and intelligence agencies in this case.
The murder is the first fatal Islamist attack in Britain since 2005, when suicide bombers killed 52 people on London's transport network.
Counter-terrorism police conducted searches near the murder scene on Thursday, and confirmed that they had raided an address in Lincolnshire, eastern England.
In the video, which has gone viral on the Internet, the man reported to be Adebolajo makes various political statements including a demand for Cameron to "bring our troops back".
Speaking in a London accent he tells the camera: "I apologise that women had to witness this today, but in our lands our women have to see the same.
"You people will never be safe. Remove your governments, they don't care about you."
Shocked eyewitnesses described how the men stayed at the scene after the killing, asking passers-by to photograph and film them.
Media reports citing witnesses said the men first ran over their victim in a car before finishing him off with the knives.
Several witnesses said he had been decapitated.
Rapper Boya Dee, who witnessed the incident, wrote on his Twitter account: "Ohhhhh myyyy God!!!! I just see a man with his head chopped off right in front of my eyes!"
The victim was wearing a top bearing the logo of Help for Heroes, a charity that helps wounded troops.
Flowers have been laid outside the barracks, which was the venue for shooting events at the London 2012 Olympics, while security was tightened at other military sites across the city.
Meanwhile a female scout leader has shot to fame after she confronted the assailants shortly after the attack, telling them: "It is only you versus many people. You are going to lose."
Ingrid Loyau-Kennett, 48, told the Daily Telegraph that when she asked one of the suspects why they had killed the soldier, he said: "We want to start a war in London tonight."
The Muslim Council of Britain has described the murder as "a truly barbaric act that has no basis in Islam".
Late Wednesday around 250 members of the anti-Islamist English Defence League were involved in minor scuffles with police in Woolwich, while two men were arrested overnight after separate attacks on mosques in southeast England.