British police made two further arrests on Thursday and raided houses across London, following the brutal murder of 25-year-old Lee Rigby, a serving soldier who survived a tour of duty in Afghanistan, AFP reported.
Police searched five properties in London and one in a village in eastern England, and announced the arrests of a man and a woman, both aged 29, for conspiracy to murder.
"This is a large, complex and fast-moving investigation which continues to develop," a spokesman said, according to AFP.
Detectives are sifting through witness statements, social media and security camera footage, while forensic experts have been combing the scene in Woolwich for evidence, the report said.
Britain’s intelligence agencies, meanwhile, came under scrutiny after it emerged that the two murder suspects, who were wounded by police gunfire at the scene, had been known to the security services.
Both men, aged 28 and 22, are believed to be Britons of Nigerian origin. One of them had frequented meetings by the now-banned Islamist group Al-Muhajiroun, its UK leader Anjem Choudary told AFP.
The two chief suspects are under arrest in separate hospitals. They are both stable and their injuries are not life threatening, police said.
In a brazen mid-afternoon attack in Woolwich, southeast London, the pair apparently hacked Rigby with knives and a meat cleaver before attempting to explain their actions in an Islamist tirade to passers-by.
The victim, who has a two-year-old son Jake, was a machine gunner who served with NATO-led forces in Afghanistan in 2009, the defense ministry said.
Prime Minister David Cameron has condemned the "barbaric" attack and said it was likely terrorist-related, while experts said the murder had the hallmarks of a Islamist terrorist attack.
Media reports citing witnesses said the men first ran over their victim in a car before finishing him off with the knives.
A government source said "we will be looking into the possibility that they were known" to the security services, according to AFP.
British media are naming one of the suspects as Michael Adebolajo, a 28-year-old Londoner.
Islamist preacher Choudary told AFP 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.
"He used to attend some of our activities over the years. Very peaceful chap actually, not violent at all," Choudary said, adding that he lost contact three years ago.
Choudary is himself a hate preacher who has said in the past he plans to flood specific Muslim and non-Muslim communities around the UK and “put the seeds down for an Islamic Emirate in the long term.”
He founded two Muslim groups in Britain that were banned by the British government after being declared terrorist organizations. He has threatened British Jews who support Israel, stating that it is an “Islamic obligation upon Muslims everywhere to support the Jihad against those who fight Muslims anywhere in the world or who occupy Muslim land.”
He has often praised Muslim terrorists, referring to the September 11 terrorists as “magnificent martyrs.” In 2003 he endorsed terrorist attacks by British Muslims and said that al-Muhajiroun, one of the groups he founded, would “encourage people to fulfill their Islamic duties and responsibilities.” He praised the 2008 terrorist attack in Mumbai, India and has also called for assassinating the Pope and prosecuting Queen Elizabeth for genocide.
Speaking after chairing a second meeting of COBRA, the government's emergency response committee, Cameron 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.
He added, "We will never give in to terror -- or terrorism -- in any of its forms... This view is shared by every community in our country."
Chilling onlooker footage showed the man thought to be Adebolajo still holding his weapons and saying they killed Rigby "because Muslims are dying daily by British soldiers."
In the video, which has gone viral on the Internet, Adebolajo demanded Cameron's government "bring our troops back".
"You people will never be safe. Remove your governments, they don't care about you," he said, according to AFP.
U.S. President Barack Obama condemned the killing as "appalling" and "horrific", saying there was "absolutely no justification for such acts."
There was shock and fury in Britain on Thursday following the gruesome murder. In the hours following the attack, around 250 supporters of the anti-Islamist English Defense League (EDL) gathered at Woolwich Arsenal train station. The group reportedly hurled bottles and became involved in minor skirmishes with the police, but later dispersed without any arrests being reported.