Investigators probing the derailment of an express train in Russia say they have found "elements of an explosive device" at the site and believe an act of terror caused the deadly incident.

The derailment killed at least 26 people and injured about 100, but there was no immediate word on who or what group might have been responsible for the action.

The investigative committee of the Russian prosecutor's office said on Saturday police found "a crater with a depth of about 1.5 metres on the railroad bed" and traces of a bomb.

"Criminology experts have come to a preliminary conclusion that there was an explosion of an improvised explosive device (IED) equivalent to seven kilos of TNT," Alexander Bortnikov, the Federal Security Service Director, said.

The Nevsky Express derailed in remote countryside on Friday night as the train travelled between the capital Moscow and St Petersburg.

Elements of an explosive device found at the scene has led to speculation about a possible terrorist attack, as Al Jazeera's Jonah Hull reports.

Source: Al Jazeera