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.

-- 'You people will never be safe' --

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.

London soldier murder raises fears of 'lone wolf' attacks
London (AFP) May 23, 2013 - The brutal murder of a British soldier on a London street has the hallmarks of a militant Islamist attack but one conducted by 'lone wolf' operators -- a security nightmare, experts said on Thursday.

Counter-terror police are investigating Wednesday's attack in which two men hacked the soldier to death in broad daylight. They were shot and wounded by police and are now in hospitals under armed guard.

The gruesome nature of the murder -- some witnesses say the men tried to decapitate their victim -- their politically-fuelled rants to passers-by, and their apparent desire for publicity all point to militant Islamist extremism, experts say.

But the small scale of the attack -- far removed from the massive destruction of the September 11, 2001 attacks on the United States and the coordinated suicide bombings in London in 2005 -- fit with fears that lone operators are now a serious threat.

"The problem with amateurs is they just do something -- and that's what makes them dangerous," John Gearson, professor of national security services at King's College London, told AFP.

"This is a departure from the established type of attacks that you see or the established plan that you see of terrorism causing mass murder," explained John O'Connor, a former head of the Scotland Yard police headquarters' 'Flying Squad' detective unit.

"The bottom line is that it could spring up anywhere and that's the concern. It's very difficult to keep a tab on where this is going and where the threat level is," O'Connor told the BBC.

Security experts have long been warning of the risk of terror attacks from individuals who slip under the surveillance radar.

Last year Jonathan Evans, the head of British spy agency MI5, warned about the threat from "lone actors" who were hard to identify because they used everyday weapons, often did not belong to any group and did not attend terror training camps abroad.

"The one thing that will be causing them (security forces) not to sleep at night is the 'lone wolf'," RUSI security analyst Margaret Gilmore told AFP ahead of the Olympic Games in London last year.

-- Attackers targeted soldier, sought publicity --

Although the investigation is at an early stage, experts have pointed to the similarities between the London murder and the acts of Islamist fanatics in extremist videos broadcast on the Internet.

Matthew Henman, senior analyst at defence group Jane's, highlighted the targeting of a soldier, and said the suspects' failure to flee the scene and tirades before filming onlookers suggests "an effort to maximise publicity of the attack".

Prime Minister David Cameron has vowed the security services "will not rest until we know every single detail of what happened and we've brought all of those responsible to justice".

He acknowledged reports that the two suspects may have been previously known to the intelligence services, but praised the work of the security forces in foiling plots up until now.

The murder was the first fatal Islamist attack in Britain since 52 people were killed in suicide bombings on the London transport system eight years ago.

Since then, surveillance has been stepped up, as have border controls, and many plots have been disrupted.

Some of these have targeted British soldiers, who were once told not wear their uniforms in public due to a threat from Irish republicans, but in recent years have been encouraged to do so.

Last month, three British men were jailed for planning terror attacks, including at Royal Wootton Bassett, a town which for years was the first to receive soldiers killed in Afghanistan.

Another pair were jailed the same month for planning terror acts including an Al-Qaeda-inspired plot to send a remote-controlled toy car into an army reservist centre.

In 2008, another man, Parviz Khan, was jailed for life for plotting to kidnap and kill a British Muslim soldier. He admitted he intended to film his beheading.