LONDON — Pressure mounted on Friday for MI5, Britain's domestic security agency, to explain how two men with years of involvement with extremist Islamic groups were left free to kill an off-duty soldier this week, striking him with their car in a London suburb and then hacking him repeatedly with butchers' cleavers.
For its sheer brutality, and the fact that the episode was recorded in detail by witnesses with cellphone cameras, the attack outside the Royal Artillery Barracks in Woolwich on Wednesday came as a shock to many in Britain after years of success by MI5 and other security agencies in tracking militant cells and pre-empting terrorist plots.
On Friday, officials confirmed the identity of the second of the two suspects, naming him as Michael Adebowale, 22, who was born in Nigeria and immigrated to Britain as a child. The other man had previously been identified as Michael Adebolajo, 28, who was born in Britain to a Christian family that moved here from Nigeria, and converted to Islam after the Sept. 11, 2001, attacks.
Both men survived police gunshot wounds sustained after the death of Lee Rigby, 25, a bandsman and machine-gunner in the Royal Fusiliers, when witnesses said they charged at police officers with cleavers and a handgun. They remain under armed police guard in London hospitals and have not yet been charged.
The possibility of wider involvement in the attack was suggested by the arrests of two women and a man believed to have family links to Adebolajo, though the women were released without being charged on Friday and only the man, said by the police to be 29 years old, remained in custody.
Officials confirmed Friday that the two suspects in the killing had been known to MI5 for years, and in the case of Adebolajo, since at least 2005. While there were few details about the militant activities of Adebowale, Adebolajo has a long record of involvement with extremist groups, the New York Times reported, citing British security officials.
Those activities, they said, included involvement in violent protests and an arrest at a London airport last year when he was preparing to fly to Somalia to join the Shabaab, a group linked to al-Qaida that has been listed as a terrorist group by Britain and the United States.
Prime Minister David Cameron said Thursday that the Intelligence and Security Committee would review the work of agencies such as Britain's domestic spy agency, MI5, in the wake of the attack "as is the normal practice in these sorts of cases."
On Friday, British newspapers published photographs of Adebolajo at several protests organized by extremist Islamic groups in recent years, including an image showing him at a London protest in 2007 standing behind Anjem Choudary, leader of Al Muhajiroun — "the Emigrants'' in Arabic — a group that was banned in 2010. The ban came after relentless support by the group's leaders for jihadist activities, including attempts to disrupt parades and ceremonies honoring soldiers returning from Iraq and Afghanistan and for its public celebrations of the Sept. 11 terrorist attacks.
Wednesday's attack was captured on video by passers-by and made for gruesome viewing — one man is seen with his hands stained red with blood and holding two butcher's knives as he angrily complained about the British government and troops in foreign lands.
Analysts say the attackers wanted the publicity to inspire copycats. Already, there has been increased chatter on militant sites, they said.
"We can see the tempo being raised," said Maajid Nawaz, a former jihadist who is now with the London-based anti-extremist Quilliam Foundation. "One of the reasons why these guys acted in this theatrical way was because of the propaganda effect so others would be inspired to do the same thing."
Amid widespread calls for an official accounting from MI5 and other security agencies, Rigby's family held an emotional news conference in Manchester, his hometown. They described him as a vibrant, well-loved man and a doting father to his 2-year-old son, Jack.
His stepfather, Ian Rigby, flanked by the soldier's sobbing mother and wife, broke down as he read a eulogy to the serviceman, who had served in a combat unit in Helmand, the Afghan province that has been the scene of much of the bloodiest fighting of the war by British and American troops.
"When he's in the UK, you think they're safe," said Rebecca Rigby, who was so overcome with emotion at times she could barely speak. "He's walked up and down that road so many times before."
"When he's in (Afghanistan), you come to terms with it. You know there's dangers," added Ian Rigby. "You don't expect something like that on your doorstep. It's very difficult."
The incident has raised fears not just of copycat violence but also of a backlash against Muslims in Britain. Hours after the killing, members of the far-right English Defense League converged on the attack site to rally against Islam, clashing with riot police. Several mosques have reported vandalism and other abuse since Wednesday's slaying.
Religious and political leaders have called for tolerance and restraint.
"This attack on a member of the armed forces is dishonorable, and no cause can justify this murder," Ibrahim Mogra, an imam with the Muslim Council of Britain, said Friday after a meeting with Christian leaders, including Archbishop of Canterbury Justin Welby. "This crime has heightened tensions throughout the country.''
In Woolwich, where Rigby was stationed, dozens of bouquets of flowers covered the sidewalk near where he died.
Ian Rigby said that his stepson's last text message to his mother read: "Thank you for supporting me all these years. You're not just my mum; you're my best friend. Good night. I love you loads."
"We would like to say, 'Good night, Lee,' " Ian Rigby said. "Rest in peace, our fallen soldier."
Information from the New York Times, Los Angeles Times and Associated Press was used in this report.