The device exploded with the force of 15 pounds of TNT as a popular luxury express train, the Nevsky Express, passed over it Friday night in a wooded area about 200 miles northwest of Moscow. The blast threw at least three carriages from the tracks, injuring as many as 100 passengers and leaving a 5-foot-deep crater, officials said.

Russian authorities named no immediate suspects or motive, but the investigation was expected to focus on Muslim radicals who have stepped up attacks this year in a separatist insurgency in the volatile North Caucasus region, including Chechnya and neighboring Ingushetia and Dagestan.

If their involvement is confirmed, the attack on the Nevsky Express would mark a bold escalation by rebels the Kremlin and its main ally in the region, the Chechen strongman Ramzan Kadyrov, insist are on the run and all but defeated — and a potential return to the violence that terrorized Moscow and other Russian cities in the first half of the decade.

Though shootings and bombings are a daily occurrence in the North Caucasus, more than five years have passed since the militants last staged a deadly strike outside the region. The rebel leader, Doku Umarov, issued a video in April, however, declaring civilians legitimate targets and vowing a fresh wave of violence across Russia.

Earlier this week, a man identified by prosecutors as a follower of Umarov’s, Mapsharip Khidriev, admitted in a regional court that he played a role in a 2007 bombing that derailed the same train traveling the same route, according to local news reports. There were no fatalities, but more than 27 people were injured in that attack.

“All search and investigative measures must be conducted intensely and most thoroughly,” President Dmitry Medvedev told senior officials in a nationally televised emergency conference. “You need to act quickly so as to find as much evidence relating to the case as possible. . . . The results of the investigation must be reported to me personally.”

A second, smaller blast caused by another bomb that partly exploded briefly disrupted rescue operations Saturday afternoon, but no one was hurt, said Vladimir Yakunin, the Russian Railways chief.

Boris Grost, a passenger in the eighth of the train’s 14 cars, said he heard an explosion to the right of the train before suitcases suddenly began falling from the shelves above their seats.

“There was a feeling that either the train lost one of the wheels or had ran over something,” he told the state broadcaster TV Center. “For 30 minutes, we didn’t know what was happening until the conductor told us three carriages had gone off the rails.”

“The first one flew off the tracks, and everything in it, people, luggage and seats were crushed,” another passenger, Aleksei Pavyluts, told Vesti television from the hospital where he was being treated. “My seat was in the back, so I’m somewhat okay.”

Initial reports on the death toll varied from 26 to 30, with several other victims said to be in critical condition. Authorities confirmed that two senior government officials were among those killed, Boris Yevstratikov, the chief of the federal reserve, and Sergei Tarasov, chairman of the state road agency.

The Nevsky Express is popular with government officials and businessmen traveling between Russia’s two largest cities. The train was carrying nearly 700 passengers and railway employees to St. Petersburg when the explosion occurred at about 9:30 p.m., officials said.

After the 2007 attack on the train, the authorities initially blamed nationalists and arrested two suspects. But Alexandor Belov, coordinator of the Movement Against Illegal Immigration, a prominent nationalist group, said the government was trying to downplay the possibility of Chechen involvement in both derailments “to avoid inflaming hatred against Chechens.”

Muslim radicals and ethnic separatists from the North Caucasus have targeted trains in the past. At least a dozen people were injured when a bomb caused the derailment of a train heading from Chechnya to Moscow in June 2005, and a suicide attack on a commuter train near Chechnya killed 44 people in December 2003. Chechen rebels were also blamed for the February 2004 subway bombing in Moscow that left 40 people dead.

Researcher Alyona Molchanova in Moscow contributed to this report.