Like the United States only a month ago, Britain on Thursday was once again grappling with an all-too-familiar plague — homegrown terrorism.

The images of two men captured on amateur video, bloodied and ranting after striking a British soldier with a car and hacking him to death on a southeast London street, became seared into the British consciousness on Wednesday. A day later, British officials investigating the case arrested two other people — a man and woman, both 29 — on suspicion of conspiring to commit murder.

In this multicultural nation that has stood alongside the United States in Iraq and Afghanistan since the attacks of Sept. 11, 2001, the population was processing the news that both alleged assailants in Wednesday’s attack — ages 22 and 28 — were fellow Britons. One of them, Michael Adebolajo, a British man of Nigerian origin, had grown up in a devout Christian family in suburban London and converted to Islam a decade ago, according to the BBC.

Social media buzzed with unverified characterizations from those who claimed to know him. They portrayed Adebolajo as a rambunctious, if regular, guy who appeared to slip into Islamist extremism. “How could someone who was a laugh and nice bloke at school turn out like that,” tweeted Paul Leech, who identified himself as a former classmate.

The alleged co-attacker remained unnamed, but domestic media reports suggested that he was a Nigerian-born naturalized Briton. London’s Metropolitan Police and counterterrorism agencies are conducting a large-scale investigation, including raids on six homes in London and the city of Lincoln, with 1,200 extra police officers being deployed in the British capital as a precaution.

But for many, the challenge Thursday was coming to grips with what was only the latest in a string of homegrown terrorist incidents here. In particular, the bizarre images of one attacker, who looked into cellphone cameras and proclaimed that Wednesday’s slaying was an “eye for an eye” for British violence “in our lands,” raised another question on both sides of the Atlantic: How could two men feel so alien in their own country?

“We saw it in Boston, and we saw it here on Wednesday. There is no such thing as a typical terrorist profile,” said Michael Clarke, director general of the Royal United Services Institute, a London-based defense and security think tank. “But these guys were like the Boston bombers in the respect that they were both what I could call homicidal exhibitionists, a sort of Bonnie and Clyde team wanting to get attention and ready to go down fighting.”

The risk of radicalization

Britain has experienced domestic extremism. In February, three men from Birmingham were convicted in a courthouse near the site of Wednesday’s attack of plotting suicide bombings to rival the 2005 London transit blasts that killed 52 people.

That coordinated attack was carried out by four homegrown suicide bombers. Concern also is growing here over a trickle of British citizens who have joined the fight in Syria against the government of Bashar al-Assad, with some fearing their exposure to religious extremists among the opposition forces.

In the late 2000s, Britain launched an engagement program that recruited Muslim mentors to work with those of their faith who appeared to be at risk of becoming radicalized by extremist clerics and propaganda. ­Government-funded community groups also have engaged radical clerics on the ground and sent outreach workers into gyms, prisons and neighborhood streets on a quest to combat extremism.

“Everyone is horrified and shocked, and emotions are raw right now,” Atif Iqbal, a Muslim community outreach worker in Birmingham, said of the attack. “It’s frustrating and so unfortunate that people like this can just slip through.”

One radical cleric whom Iqbal and others in Birmingham confronted two years ago, Anjem Choudary, told the Independent newspaper that Adebolajo had once regularly attended his lectures. Choudary, the former leader of a now-banned Islamist group, referred to Adebolajo by his adopted name, “Mujahid,” or Muslim warrior.

“He was a pleasant, quiet guy,” Choudary told the Independent. “He reverted to Islam in about 2003. He was just a completely normal guy. He was interested in Islam, in memorizing the Koran. He disappeared about two years ago. I don’t know what influences he has been under since then.”

‘A betrayal of Islam’

The suspects, who were shot while being apprehended by police, were under guard at separate hospitals. Late Thursday, Britain’s Defense Ministry released the name of the slain soldier, drummer Lee Rigby of the 2nd Battalion of the Royal Regiment of Fusiliers. Rigby, a 25-year-old father of a 2-year-old boy named Jack, was described by his commanding officer, Lt. Col. Jim Taylor, as “a real character” and “true warrior.” Taylor said Rigby had served with distinction in Afghanistan, Germany and Cyprus.

Prime Minister David Cameron on Thursday said Wednesday’s killing 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 cautioned against “knee-jerk responses.”

Cameron appeared to acknowledge reports that both men were known to British security services and that one had attempted to travel to Somalia to support an al-Qaeda affiliate. He said there would be reviews of British security services’ management of any information that had been received about either man in recent years.

“The people who did this were trying to divide us,” Cameron said. “They should know that something like this will only bring us together and make us stronger.”

Eliza Mackintosh contributed to this report.