LONDON (CNN) -- British police have arrested two more people as part of the probe of the brutal cleaver attack that killed a British soldier.
A man and a woman, both 29, were arrested Thursday on suspicion of conspiracy to murder, Metropolitan Police said. No further details wee given.
Warranted searches have been carried out at six residential addresses, police said.
"This is a large, complex and fast-moving investigation which continues to develop," police said in a statement. "Many lines of inquiry are being followed by detectives and the investigation is progressing well."
Wednesday's attack left London reeling and raised questions over whether Islamist extremism was to blame.
The scene revealed through cell phone camera footage and witness accounts was bloody, and for many hard to believe.
A meat cleaver-wielding man with bloody hands addressed a camera, his victim lying mutilated in the street behind him.
"The only reasons we killed this man ... is because Muslims are dying daily," he said in video aired by CNN affiliate ITN.
"This British soldier is an eye for an eye, a tooth for tooth," said the man, who independent sources and British media identified as Michael Adebolajo. "We swear by almighty Allah we will never stop fighting you until you leave us alone."
CNN has not independently confirmed the name.
The two men arrested at the scene are being treated in South London hospitals for injuries suffered in a confrontation with armed police in southeast London's Woolwich neighborhood. Both were in stable condition on Thursday evening, the Metropolitan Police tweeted.
Authorities are calling on the public to send any videos or photos of the incident.
The slain soldier has been named as Lee Rigby, who held the rank of drummer, the Ministry of Defense said.
The crisis brought Prime Minister David Cameron rushing back Wednesday night from an official trip to Paris.
He condemned the "horrific attack" in a news conference Thursday, vowing that Britain would be "absolutely resolute" in the face of terrorism.
"We will never give in to terror or terrorism in any form," he said.
Cameron said Britain's security services would not rest until they brought all those responsible to justice.
U.S. President Barack Obama issued a statement Thursday condemning the attack.
"The United States stands resolute with the United Kingdom, our ally and friend, against violent extremism and terror," the statement said. "There can be absolutely no justification for such acts, and our thoughts and prayers are with the family of the victim, the police and security services responding to this horrific act and the communities they serve, and the British people."
Obama said that the "special" U.S.-British relationship "is especially important during times of trial."
It is understood that the two people suspected of carrying out the knife attack were known to Britain's domestic security service. They had featured in previous investigations into other people, but were not themselves under surveillance.
Amid concern over a backlash against the Muslim community, Cameron insisted the slaying had nothing to do with Islam, despite the claims made by one of the suspected attackers.
He spoke after a crisis meeting of senior officials Thursday morning, as security was increased at army bases around London amid fears of additional attacks.
The calling of the crisis meeting -- the second in less than 24 hours -- indicates how seriously the government is taking what it says is a terrorist incident.
The capital has not witnessed an alert of this kind since the summer of 2005, when London's public transport network was targeted with coordinated bomb attacks.
As the initial shock over the man's gory death at the hands of two knife-wielding men ebbed Thursday, an individual story of courage stood out.
A Cub Scout leader, Ingrid Loyau-Kennett, told Britain's Daily Telegraph newspaper how she had jumped off a bus to try to revive a man she at first thought had been hurt in an accident.
But she swiftly realized the man was dead, and it was not an accident.
"When I went up, there was this black guy with a revolver and a kitchen knife. He had what looked like butcher's tools, and he had a little axe, to cut the bones, and two large knives, and he said, 'Move off the body,' " she told the newspaper.
"So I thought 'OK, I don't know what is going on here,' and he was covered with blood. I thought I had better start talking to him before he starts attacking somebody else."
A man who identified himself as James told London's LBC 97.3 radio station that he saw two men standing by the victim, who was on the ground.
At first, James thought they were trying to help the man. But then he saw two meat cleavers, like a butcher would have.
"They were hacking at this poor guy, literally," he told the radio station. "These two guys were crazed. They were just not there. They were just animals."
Another witness, Michael Atlee, described the gruesome, frenzied and ultimately fatal sequence of events as "a bloody mess." The men first ran the victim down in a car before attacking him with knives, he said.
Searches have been carried out at several locations in connection with the slaying, Assistant Commissioner for Specialist Crime and Operations Mark Rowley said. One address searched was in Lincolnshire, eastern England.
An additional 1,200 police are on London's streets to reassure the public, Rowley said, with extra patrols at key locations such as religious institutions and transport hubs.
"We are also liaising closely with the military to ensure appropriate security measures are in place in relation to military premises and personnel," he said.
The scene of the killing, close to the Royal Artillery Barracks in Woolwich, remained cordoned off as police searched for clues Thursday morning.
'People will break'
Residents on Thursday shared with CNN their shock that something like this could have happened in the working-class, multicultural area where they live and work.
Construction worker Victor Easdown, who heard the shots ring out as police took on the attackers, fears the incident could fuel tensions and reprisal attacks.
"People can only take so much. And people will break," he said.
Graham Wilder, a resident whose son attends a nearby school, told how he feared for the safety of his family and other children who had just left the school Wednesday afternoon.
After he saw that one of the attackers had a gun, he alerted police and school authorities, Wilder said. He heard shots fired and screamed for his wife, who was at a nearby store, to get down.
Cameron said Britons would stand together to defeat the threat of violent extremism.
"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.
"There is nothing in Islam that justifies this truly dreadful act. ... The fault lies solely with the sickening individuals who carried out this attack."
Prominent British Muslim radical leader Anjem Choudary told CNN on Thursday that he knew one of the men named on social media as carrying out the Woolwich knife attack.
Choudary said the suspect had attended demonstrations and a few lectures organized by Choudary's group Al-Muhajiroun.
There were concerns the brutal incident might inflame animosity against Muslims, with Metropolitan Police deploying riot police Wednesday as a precautionary measure.
The Muslim Council of Britain, after condemning what it called "a truly barbaric act that has no basis in Islam," urged Muslims and non-Muslims alike "to come together in solidarity to ensure the forces of hatred do not prevail."
"What we have seen on the streets of London has been particularly sickening, a really, really heinous act of I would say criminality -- and I'm being careful to say criminality, not terrorism," political and social commentator Mohammed Ansar told CNN.
The motivation behind what happened remains unclear, he pointed out.
"What we need at this time is a sense of calm, a sense of measure and a sense of perspective. What we don't need are knee-jerk reactions ... to really ratchet up tensions and really stoke and inflame anxieties within communities."
Members of the far-right English Defense League clashed with police late Wednesday.
The group's official Twitter account posted this call to action: "ANY EDL MEMBERS TAKE TO THE STREETS IN YOUR LOCAL TOWN/CITY TAKE A STAND !!!!!!"
Later Wednesday, a man with two knives threw a smoke grenade into a mosque in Essex, a county east of London, and demanded someone come outside to answer to the Woolwich slaying, the mosque's secretary said. Police responded quickly and arrested the man, said Al Falah Braintree Islamic Center secretary Sikander Sleemy.
"I believe this was a revenge attack for what happened in Woolwich," Sleemy said. "We strongly condemn what happened in Woolwich. It's not an Islamic act."
In Kent, police arrested a man on suspicion of "racially aggravated criminal damage" at a religious building.
Soldiers targeted before
Nick Raynsford, the member of Parliament for Woolwich, told CNN the soldier apparently had been on duty in central London and was returning to the barracks when he was attacked.
Troops stationed at the historic military barracks have a close relationship with locals, the Parliament member said.
British soldiers have participated in the wars in Afghanistan and Iraq. They are due to withdraw from Afghanistan by the end of 2014.
This isn't the first time British soldiers have been singled out.
Last month, four radical Islamists were convicted at Woolwich Crown Court of a plot to drive a car full of explosives, by remote control, into an army barracks in Luton, north of London.
Several years earlier, police interrupted a scheme in which Islamists planned to kidnap a soldier of Pakistani heritage and behead him. Their plan called for releasing an Internet video of the decapitation.
A pub in the same area of Woolwich was targeted by the Irish Republican Army in 1974. Two people died in the bombing.
Local residents said police responded quickly when the alarm was raised Wednesday afternoon but questioned how long it had taken for a specialist firearms unit to arrive. British police typically don't carry guns.
The Metropolitan Police said its first officers were on the scene within nine minutes of the alert being raised. The firearms unit was there 14 minutes after the first call was made, the force said.
"There has been an increased police presence in Woolwich and the surrounding areas overnight, and this will continue for as long as it is needed," said Assistant Commissioner Byrne.
"There were small incidents of minor disorder in Woolwich" late Wednesday, he said, but police dealt with these without arrests or reports of injuries or damage.