The British security authorities have contained the threat of jihadist-inspired terrorism since the London bombings of July 2005 that killed more than 50 people. But the butchering of a soldier outside his London barracks this week by two men who linked their actions to Britain's military role in Afghanistan and urged bystanders to overthrow the government has, understandably, left the public shocked and unnerved.
Without more detail, it is not clear whether the men, said to be Christian converts to Islam, were acting entirely alone. But the outrage raises worrying questions about whether home-grown extremism has evolved into a threat that is harder to counter than plots involving organisation, training, arming and the use of explosives.
A free society - a multicultural one, moreover - can never be completely secure from terror as long as the threat continues. Counterterrorist intelligence and policing can keep people as safe as possible from network attacks. But individuals operating independently, as appears to have been the case, pose a new challenge. It calls for the redoubling of efforts to deepen engagement with multicultural communities.
The two suspects are in custody in hospital after being shot by police. If their aim was to inflame tensions between communities - a serious danger of such outrages - they have succeeded. Within hours, dozens of Islamophobic crimes and incidents were reported, including attacks on mosques. Justice for a slain husband and father selected at random from Afghanistan veterans - in a fair trial for the accused under a way of life they attacked - is a better answer to terrorism.
Prime Minister David Cameron vowed that Britain would not be cowed by extremist violence. His description of the killing as a betrayal of Islam as well as an attack on Britain has been echoed in condemnations by the main Muslim groups. He has rightly resisted calls for a security crackdown. Terrorism feeds on overreaction to attract sympathy and new recruits.