THE senselessness of murdering and maiming innocent people indiscriminately is the most shocking aspect of terrorist attacks.

That was especially true of the bombs which killed and injured runners and spectators, including children, at the Boston marathon on Monday.

Motive was at first as elusive as the perpetrators but the identification of brothers Tamerlan and Dzhokhar Tsarnaev (on the death of Tamerlan following a shoot-out with police) as chief suspects suggests the attack was an Islamist strike against American values, possibly also in support of radical Chechen insurgents. The Tsarnaevs moved to the US in 2001 with their parents from Dagestan, a North Caucasus republic neighbouring Chechnya (which they fled in the late 1990s during the second Chechen war). Since they spent the last decade in America, it is difficult to determine whether to categorise the Boston bombs as home-grown terrorism.

But as "clean skins" with no known links to terrorist organisations they illustrate the scale of the problem for the security services. A comparison with a needle in a haystack is apt.

The younger brother, Dzhokhar Tsarnaev, now 19, went to school in the US and was awarded a scholarship for further education in 2011. He appeared to have been absorbed into American society, showing no signs of being a radical Islamist to those who knew him as a student. His father, who has returned to Dagestan, said he expected him to become a doctor. It is particularly disturbing when those whose professional motivation is to save lives are discovered to be intent on slaughter but, as we learned with the attempt on Glasgow Airport in 2007, doctors can also be terrorists. Dzhokhar Tsarnaev posted links to videos of fighting in the Syrian civil war and pages supporting Chechen independence on his social networking sites. An interest in these issues is to be expected from someone of his background, but does not make him a terrorist. Unlike most attacks carried out under the umbrella label of jihad, this was not a suicide bombing. Both brothers walked away.

That leaves the question of whether, as ethnic Chechens, their motive was related to Chechnya. The elder brother Tamerlan appears to have been more radicalised and interested in jihadi videos. It also seems he found the transition to life in the US more difficult. In his teens he told an interviewer that he did not have a single American friend because he did not understand them.

If the Tsarnaevs' motive was inspired by Chechen radicals, an attack in the US would set a new precedent in the continuing conflict with the Kremlin but some Chechens have become foreign fighters in Syria and it might appear to Chechens in the US that they could best protest against American policies in the Middle East by direct action, however vile.

There may eventually be an explanation from Dzhokhar Tsarnaev but security services, not only in the US, know that a generation with the power of the internet at their fingertips poses a new and formidable challenge.