As many as 100 people were left injured by the disaster, officials said, and 18 remained missing Saturday night.
"Indeed, this was a terrorist attack," said Vladimir Markin, a spokesman for the investigative committee of Russia's General Prosecutor's Office, according to Interfax. Bortnikov said the bomb exploded with a force equivalent to 15 pounds (7 kilograms) of TNT.
The attack on the Nevsky Express, a luxury train popular with government officials, tourists and business executives, was Russia's deadliest terrorist strike outside the volatile North Caucasus region since 2004.
Among the dead were citizens of Belgium, Italy and Azerbaijan, Governor Valentina Matvienko of St. Petersburg told the state-run RIA Novosti news agency.
The force of the derailment scattered battered carriages over a remote stretch of track, trapping some of the injured for hours. By first light Saturday, luggage and pieces of metal lay in the muddy embankment in the drizzle, as survivors huddled under blankets by fires.
A second explosive device partially detonated at the crash site later Saturday as railway workers tried to clear the debris, said Russian Railways chief Vladimir Yakunin.
The initial blast derailed the last three carriages of the 14-car Nevsky Express as it approached speeds of 200 kilometers per hour (130 mph), officials said. More than 650 passengers and crew were aboard at the time.
The crash occurred near the border of the Novgorod and Tver provinces, some 250 miles (400 kilometers) northwest of Moscow and 150 miles (250 kilometers) southeast of St. Petersburg.
As rescue workers searched for victims, officials provided sometimes conflicting numbers for the dead and injured.
Health Minister Tatyana Golikova said at least 26 people were killed, 18 were missing and nearly 100 injured in the derailment. The Prosecutor General's office said the death toll had risen to 30, with 60 others in the hospital.
As of Saturday evening, there were no credible claims of responsibility. But sketches were being composed of several suspects, Interior Ministry chief Rashid Nurgaliyev told Interfax, including of a man of about 40 with red hair.
Witnesses said the train left the tracks after a loud explosion.
"It was immensely scary. I think it was an act of terrorism because there was a bang," passenger Vitaly Rafikov told the state-run Channel One television network. He said he helped haul victims from the battered carriages and lit fires to keep them warm.
Passenger Igor Pechnikov was in the second of the train's three derailed cars. "A trembling began, and the carriage jolted violently to the left. I flew half way down the car," he said.
The injured were transported to hospitals in Moscow and St. Petersburg by bus, train and even helicopters, but some said the evacuation was agonizingly slow.
Yekaterina Ivanova, a wounded passenger, told the NTV television network that workers took at least four hours to free her from the wreck.
"In the hospital, the doctors are better, the medical teams are working in harmony," she said. "The young people from the Ministry of Emergency Situations carried us out on stretchers, but other people in uniform were just standing there and staring, and no one was even helping to carry out the wounded."
Russia has been hit by a number of major terrorist attacks since the 1991 Soviet collapse, including the seizures of the Dubrovka theater in October 2002 and the taking of more than 1,000 hostages in Beslan's School Number One in September 2004.
Both were staged by Muslim separatists from Russia's North Caucasus, who also claimed responsibility for the bombings of the Moscow subway, a rock concert and two airliners.
The number of terrorist attacks declined along with the scale of fighting in the North Caucasus after about 2005, as the Kremlin consolidated control of the region.
But over the past year, the Caucasus has seen a surge in violence, including an increase in suicide bombings, assassinations and attacks by militants against police and government officials.
At the same time, rights activists say, government security services in the region have stepped up the use of kidnappings, extrajudicial killings and home-burnings against suspected militants and their relatives. The Moscow-based rights group Memorial issued a report this month accusing authorities of implementing "a policy of state terror."
While Russians have grown accustomed to hearing about mayhem along their southern border, the attack on the Nevsky Express appeared to strike a nerve, inspiring massive coverage by state-run and other media.
President Dmitry Medvedev called Saturday for calm. "We need there to be no chaos, because the situation is tense as it is," he said.
Military analyst Pavel Felgenhauer told APTN that Islamist separatists who operate in the North Caucasus and nationalist groups would naturally fall under suspicion.
One prominent nationalist group, the Movement Against Illegal Immigration, issued a denial of responsibility Saturday.
Nationalists were blamed in a similar blast that caused a derailment along the same line in 2007, injuring 27 passengers. Authorities arrested two suspects in the 2007 train blast and are searching for a third - a former military officer.