The funerals of some of the 26 people who died in a blast that derailed a Moscow to St Petersburg luxury train on Friday have taken place in Russia.
Officials say the Nevsky Express derailment was caused by a bomb, but no group has yet claimed responsibility.
Meanwhile, Russian officials say a second blast which exploded on Saturday at the scene of the original blast was intended to hurt investigators.
Some analysts have suggested North Caucasus rebels may be responsible.
However a senior senator with expertise in the Caucasus, Alexander Torshin, has questioned that theory.
Mr Torshin said local "saboteurs" could be responsible for the blast, Russia's Interfax news agency reported.
"The blast was carried out professionally, we are talking about well-prepared, well-trained people using a cold-blooded calculation, and it is the counterintelligence's task to track them down," he said.
Police have identified a house where they believe suspects had been staying, Russian media reports say.
Among those who were buried on Tuesday were Sergei Tarasov, a former St Petersburg vice governor and Boris Yevstratikov, head of the Federal Agency for State Reserves.
Some of the 26 have already been buried.
Russian railways chief Vladmir Yakunin said extra security measures were being taken to prevent other similar attacks.
Police are continuing to comb the debris of the Nevsky Express - a luxury high-speed train which travels between Russia's two main cities.
Police said an "improvised explosive device" derailed the last three carriages of the express.
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. Russian media reports have put the death toll at 27, but this has not been confirmed.
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.
A second, less powerful device went off on Saturday near the site of the first, reportedly injuring one of the men searching for clues.
Alexander Bastrykin, a lead investigator, is thought to have suffered a head injury.
This explosion was reported to have been triggered by a remote mobile phone.
The Nevsky Express is an expensive train popular with government officials and business executives.
Russian officials have released a photo-fit of a man they believe is linked to the bombing.
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.
In 2007, a bomb on the same line derailed a train, injuring nearly 30 passengers.
That blast has been blamed on Chechen separatists.