The macabre attack on a British soldier on a street in southeast London on Wednesday and the equally horrific and senseless slaughter of spectators at the Boston Marathon last month both appear to have been carried out by the security services' most elusive and dangerous enemy - young lone wolves.

The blood-soaked fanatics in southeast London were apparently known to security services. Early evidence is that one of the assailants, meat cleaver in hand, cried "Allahu akbar" or "God is great." There are twin fears. Either they carried out this despicable act as part of an organized terrorist group. Or perhaps even worse than that, they carried out this act without the backing of a terrorist group. Either way, this vicious individual nature of the attack takes the fight against terrorism into uncharted territory.

Britain's Ministry of Defence said the victim was Lee Rigby, 25, of 2nd Battalion The Royal Regiment of Fusiliers.

Lone wolves are almost impossible for intelligence agencies to detect, even if they had previously shown up on their grids as potential terrorists. This is because in a free society it is immensely difficult to closely monitor everyone who might one day commit murder and impossible to charge anyone with a crime for perhaps thinking about it, or to get a search warrant to raid homes or work places based solely on the medieval ideas such people may carry in their heads. Even if there has been loose talk of committing murder, that still might not be enough evidence to get a conviction.

One of the reasons that jihadis have used the web and sermons by hard-line mullahs to encourage lone wolves to carry out acts of what appear to be random violence against people they don't know is precisely because western intelligence services have done an exceptional job of protecting prime targets and thwarting organized terrorist groups such as al-Qaida since the 9/11 attacks on New York's twin towers and the Pentagon.

But the billions of dollars and millions of man hours that have been devoted to thwarting organized jihadis are a tiny fraction of the money and time required to monitor potential solo terrorists and understand their motives, or to protect soft targets such as people gathered to watch a foot race or a soldier walking near a military base.

The immediate fear must be whether the massive publicity that the alleged killers in Boston and in the London suburb of Woolwich have received for their small part in the holy war against western civilization will inspire copycat killers. Certainly one of the questions Scotland Yard and MI5 - the domestic intelligence service charged with protecting the country from terrorism - must be urgently looking at now is whether the four suspects charged with murder in Woolwich were inspired by the recent violence in Boston, or were motivated or assisted by jihadis in Britain or elsewhere. One of the suspected attackers is a Muslim convert who participated in demonstrations with the banned radical group al-Muhajiroun, The Associated Press reported.

Immigration policies have helped to create this problem. Huge numbers of immigrants from countries with vastly different ideas about religion, democracy, justice and the place of women in society have been welcomed in western Europe, Canada, the United States and Australia.

It must be stressed that almost all of these immigrants are not and never will be supporters or agents for radical Islam. But the movement of people has been so big - in the tens of the millions - that it was inevitable that more than a few of these transplants would bring with them ancient beliefs from the old country and the will to act upon them, and that some of their offspring would feel so isolated in their new surroundings that they would seek grotesque ways to empower themselves.

But immigration is actually the smaller part of a hydra-headed problem. Tourism is an engine that powers many cities including London and the fastest growing airlines in the world are almost all in the Middle East. Every single day more than 10,000 passengers arrive in Britain on flights from the Middle East and South Asia. Thousands more land in Canada and the U.S.

Another hugely complicating factor is that there are disaffected youths who convert to radical Islam precisely because of its violent appeal.

What is clear is that a new front in the war against terrorism has opened. Political leaders must not simply peddle the message that such attacks are isolated.