Leading article: Terrorism returns to Russia

We have become all too grimly accustomed to the term "improvised explosive device" as used to kill foreign troops in transit in Afghanistan and Iraq. But in Russia? It is just such a bomb that the Russian authorities say blew up in the path of a packed Moscow-St Petersburg train, which killed 25 people and injured more than 100. Assuming – and there is no reason to doubt this – that initial reports of a bomb are confirmed, this is the first fatal act of terrorism in the heart of Russia for more than five years.

Although the war in Chechnya is over, the peace – enforced by the quixotic leader, Ramzan Kadyrov – remains uneasy. Killings and kidnappings happen, and violence periodically spills into other republics of the North Caucasus. Russia proper, though, had been spared and security had become less of a preoccupation. The explosion that derailed the Moscow-St Petersburg express could have two, highly negative, effects.

Those who planted the bomb may or may not have targeted this train. But the Nevsky Express is not just any train, it is one of the newest, fastest trains on this prime route. It is as though the IRA in its time had blown up The Flying Scotsman two thirds of the way to Edinburgh, with equivalent loss of life. It will, at a stroke, reinforce Russian feelings of national pride, national solidarity and national paranoia. Russia could become even more prickly to deal with as a result.

The second consequence is for inter-ethnic relations within Russia. While both President Medvedev and the head of the country's security service, said the right things with commendable speed, what happened to the Nevsky Express is bound to revive simmering domestic tensions. Whoever is ultimately blamed – and suspects include Chechen militants, groups from elsewhere in the North Caucasus with a grievance, or people associated with the xenophobic Russian far right – resentment will be inflamed between ethnic Russians and people from the Caucasus and Central Asia.

At a time of severe economic downturn and acute competition for jobs, the temptation to find scapegoats will be great. A bomb on the Moscow-St Petersburg line leaves questions, but so does what happens next.