British police have made two further arrests and raided houses across London following the brutal murder of a serving soldier who survived a tour of duty in Afghanistan.
A man and a woman, both 29, have been charged with conspiracy to murder.
Prime Minister David Cameron appealed for calm after 25-year-old Lee Rigby was hit with a car then hacked to death by two people outside a London army barracks on Wednesday afternoon (local time).
An extra 1200 officers have been deployed on the capital's streets in a bid to reassure the public. Intelligence agencies meanwhile came under scrutiny after it emerged that the two murder suspects, who were injured in police gunfire at the scene, had been known to the security services.
The BBC reports senior government sources confirmed that one of the suspects was intercepted by police in 2012 while leaving Britain.
The pair, aged 28 and 22, are believed to be Britons of Nigerian origin. One had frequented meetings by the now-banned Islamist group Al-Muhajiroun, its UK leader Anjem Choudary told AFP.
In the brazen mid-afternoon attack in Woolwich, south-east London, the pair apparently hacked Mr Rigby with knives and a meat cleaver before attempting to explain their actions in an Islamist tirade to passers-by.
Police searched five properties in London and one in a village in eastern England, and announced the arrests of a man and a woman for conspiracy to murder.
Detectives are sifting through witness statements, social media and security camera footage, while forensic experts have been combing the scene in Woolwich for evidence. A government source said "we will be looking into the possibility that they were known" to security services.
Soldier was 'popular and witty'
Lee Rigby of the 2nd Battalion the Royal Regiment of Fusiliers, joined the army in 2006. He was a machine gunner who served with NATO-led forces in Afghanistan in 2009 and had a two-year-old son, the defence ministry said.
The victim, who has a two-year-old son, was a machine gunner who served with NATO-led forces in Afghanistan in 2009, the Ministry of Defence said.
In a statement, it said Drummer Rigby was an extremely popular and witty soldier, a larger than life personality within the Corps of Drums and was well known, liked and respected.
Drummer Rigby had served with distinction in Afghanistan, Germany and Cyprus.
The Sun newspaper on Friday reported that the victim's girlfriend was herself serving with the Royal Military Police in Afghanistan and was flying home after receiving the news.
The soldier's family has issued a statement paying tribute to a "loving" family man. "Lee was lovely. He would do anything for anybody, he always looked after his sisters and always protected them. He took a 'big brother' role with everyone."
British media are naming the suspects as 28-year-old Michael Adebolajo and Michael Adebowale, 22, both from London.
Islamist preacher Choudary told AFP that Mr Adebolajo was from a Nigerian family, converted to Islam in 2003 and took the name "Mujahid".
He regularly attended sermons by banned Islamist preacher Omar Bakri, the Al-Muhajiroun founder.
The man thought to be Mr Adebolajo was videoed still holding his weapons and saying they killed Rigby "because Muslims are dying daily by British soldiers".
Footage released by the Daily Mirror on Friday showed the moment that police gunned down the two suspects as they charged towards officers.
The chief suspects are under arrest in separate hospitals. Both are stable and their injuries are not life-threatening, police said.
Betrayal of Islam - PM
Prime Minister David Cameron condemned the attack as a "betrayal of Islam" and branded the murderers "sickening".
"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 outside his Downing Street office on Thursday.
Mr Cameron has said that Britain will never give in to terror or terrorism in any of its forms.
The murder is the first deadly Islamist attack in Britain since 2005, when suicide bombers killed 52 people on London's transport network.