Radical Russian nationalists are the most likely culprits in a bombing which derailed a Moscow-to-St. Petersburg train, Interfax news agency reported on Wednesday, citing a source close to the investigation.

"Detectives and investigation staff are working on several angles, however the priority lead is that representatives of extremist nationalist organisations were involved in this terrorist act," Interfax quoted the source as saying.

A spokeswoman for the Prosecutor-General's office later said however investigators were not ruling out any possible explanations.

A bomb planted on the tracks exploded on Monday night just as the train passed over it on one of Russia's busiest routes. Carriages were tipped on their sides and dozens were wounded, but no one killed.

Some Russian media speculated on Wednesday the bombing might have been an attempt to influence the outcome of next year's presidential elections, when a replacement will be chosen for outgoing President Vladimir Putin.

The 2000 presidential vote, when Putin first ran for election, was preceded by a wave of apartment bombings that killed nearly 300 people. Officials linked the blasts to separatist rebels from the southern Russian region of Chechnya.

BOMBS

Radical nationalists are most commonly associated with street attacks on dark-skinned migrant workers, but they have been linked in the past to bomb attacks.

Two nationalists were convicted of blowing up a train in 2005 that was travelling to Moscow from Chechnya's capital Grozny. Eight people were wounded.

Prosecutors last year charged two students alleged to be linked to racist groups with blowing up an outdoor market in Moscow that was packed with immigrant traders. Ten people were killed.

A retired army colonel who police said had nationalist sympathies was charged in 2005 with a failed attempt to assassinate Anatoly Chubais, a former government minister widely disliked by ordinary Russians for his liberal economic reforms.

But the director of the SOVA centre in Moscow, which monitors the activities of far-right groups, was sceptical of the idea nationalists were to blame for the train attack.

"This is rubbish. There is no sense in this. When some nationalists blew up the Grozny-Moscow train that was understandable because it was full, or nearly full, with Chechens," Alexander Verkhovsky told Reuters.

"But what interest would nationalists have in the Moscow-St. Petersburg train?" he said. "In principle they are capable of doing this but there is no reason."

Police said on Wednesday they had detained a student with far-right sympathies for circulating an Internet video that appeared to show neo-Nazis executing two migrant workers -- one by beheading and the other with a bullet to his head.