Friends, acquaintances and British media have identified that man as 28-year-old Michael Adebolajo, a British national of Nigerian descent.
Police have not released his name, nor those of the others they've arrested.
Adebolajo and the 22-year-old suspect also tied directly to the attack are now hospitalized under guard after a confrontation with police in which they were shot. A 29-year-old man arrested Thursday is being on suspicion of conspiracy to murder.
Two women arrested Thursday, ages 29 and 31, have been released without being charged, police said Friday.
Police continue to search five addresses in London and one in Lincolnshire, which is in eastern England.
Suspect knew British Muslim radical leader
It is understood the two people suspected of carrying out the knife attack were known to Britain's domestic security service. They had been featured in previous investigations into other people, but were not themselves under surveillance.
Abu Baraa blamed Wednesday's attack not on his friend Michael Adebolajo -- who he says is the bloody, cleaver-wielding man shown talking in the ITN video -- but on the British government and said there may be more attacks.
"As long as (British) foreign policy is engaging in violence, they're only inviting violence in retaliation," Baraa told CNN.
By sharp contrast, Prime Minister David Cameron said "the fault lies solely with sickening individuals who carried out this attack," adding that "nothing in Islam ... justifies this truly dreadful act."
British Muslim radical leader Anjem Choudary told CNN on Thursday that he knew Adebolajo, noting the suspect attended demonstrations and a few lectures organized by Choudary's group Al-Muhajiroun.
In fact, an ITN video from April 2007 shows Adebolajo standing behind Choudary at a rally protesting the arrest of men who allegedly made inflammatory speeches inside a mosque.
Syrian cleric Omar Bakri Mohammed, who founded Al-Muhajiroun in the late 1990s, told CNN Terrorism Analyst Paul Cruickshank by phone from Tripoli, Lebanon, he also had been acquainted with Adebolajo.
Bakri Mohammed used to live in the United Kingdom but was barred from returning after the 2005 terror attacks.
Bakri Mohammed said Adebolajo, who he knew by his Muslim name "Mujahid," attended several talks he gave in London from 2003 to 2004 and was at his side at a number of Al-Muhajiroun protests against the war in Iraq around that time.
One talk Adebolajo attended was at a Woolwich community center, he said, noting the group met in such locations because they were not welcomed in mosques. The very large majority of British Muslims reject Bakri Mohammed's views.
The radical cleric said that although they did not have many interactions, Adebolajo stood out because he was a new convert to the religion.
Bakri Mohammed said he had no contact with him after he left the UK.
Volley of shots
Dramatic video footage obtained by Britain's Daily Mirror newspaper, filmed from an apartment block overlooking the street, shows the moment when armed police arrived at the scene.
One of the attackers rushes at the police vehicle brandishing knives while the other aims a gun. Both are brought down by a volley of shots.
The firearms unit was called in after the initial alert because British police do not usually carry weapons. Witnesses remarked that the two attackers appeared to wait for the armed police to arrive, nearly 15 minutes after their assault on Rigby.
The two injured suspects remained in stable condition at separate South London hospitals Friday, the Metropolitan Police said.
The attack, which Cameron and others called an act of terror, stirred anxiety and alerts in Britain not seen since the summer of 2005, when coordinated bomb attacks struck London's public transport network.
An additional 1,200 police are now on London's streets to reassure the public, police said Thursday. Extra security is in place for military personnel and sites.