A black and white sketch of the man was released on Sunday
Russian officials have released a photo-fit of a man thought to be linked to Friday's bombing of a train north of Moscow that killed 26 people.
The interior ministry circulated the sketch of a man in his 50s with a wig of red hair, who had reportedly been seen by witnesses near the blast scene.
No-one has claimed responsibility for the blast the derailed the Moscow-St Petersburg Nevsky Express.
Some analysts suggest that rebels from the North Caucasus may be responsible.
Police are continuing to comb the debris of the Nevsky Express - a luxury high-speed train which travels between the capital and Russia's second-largest city.
Police said an "improvised explosive device" derailed the last three carriages of the express.
On Monday, a bomb exploded under a train in Russia's troubled Dagestan region, but there were no casualties, police said.
Twenty five people were killed on Friday when the train derailed near the town of Bologoye, some 400km (250 miles) north-west of Moscow. The toll rose to 26 over the weekend when an injured woman died in hospital.
The rail line is a vital link between Moscow and St Petersburg
Nearly 100 people were wounded, some of them seriously. Authorities said six foreigners were among them - an Italian, a Belgian, an Azerbaijani, two Belarussians and one Ukrainian.
On Sunday, Russian Interior Minister Rashid Nurgaliyev circulated the photo-fit of a man thought to be involved in what officials say was a bomb attack on the train.
Russian officials later said the man was about 50-55 years old, about 175cm (5ft 9in) tall and wore a red-haired wig.
They also said released details about a second possible suspect, saying he was in his 30s, 180cm tall with black hair.
Investigators were also looking for a silver Zhiguli (Lada) passenger car which they believe could have been linked to the bombing, reports in the Russian media say.
The BBC's Richard Galpin, in Moscow, says the investigation is focusing on witnesses who may have seen suspicious activity near the scene of the incident.
In particular, a group of people were seen milling around the track on Friday.
Correspondents say key suspects in the investigation will be militants from the volatile North Caucasus region and extreme Russian nationalists.
The Nevsky Express is an expensive train popular with government officials and business executives.
In 2007, a bomb on the same line derailed a train, injuring nearly 30 passengers.
That blast has been blamed on Chechen separatists.