Russian police have released identikit images of three men and one woman thought to have been involved in last Friday's deadly attack on a high-speed Moscow-St. Petersburg train.

The attack on the Nevsky Express took the lives of 26 people and left over 90 injured.

"These are images of four people who were in a hideout near the crash site," north-west Russia's acting police chief, Vadim Kashirin, said.

The Nevsky Express derailed after a bomb equivalent to 7 kg (15 lbs) of TNT detonated on the track. A second, weaker bomb exploded at the site on Saturday, and is believed to have targeted investigators. No one was seriously injured.

Alexander Bastrykin, head of the Investigation Committee, told Russian President Dmitry Medvedev on Wednesday that some 400 witnesses had already been questioned and preliminary results of the investigations would be announced in 10-12 days.

The blast has raised fears of a resurge of terrorist attacks in the Russian capital and other major cities. Russia was hit hard by terrorism in the 1990s and the early years of this decade, but violence has largely been confined to the volatile North Caucasus region since 2004.

ST. PETERSBURG, December 3 (RIA Novosti)