Moscow, Russia (CNN) -- Investigators have found "elements of an explosive device" at the site of the derailment of an express train in Russia and believe an act of terror caused the deadly incident.

The derailment killed at least 26 people and injured about 100, but there was no immediate word on who or what group might have been behind the action.

"One can say with certainty that that was indeed an act of terror," Vladimir Markin, spokesman for the investigative committee of the Russian prosecutor's office, told CNN.

He would not elaborate on exactly what kind of "elements of an explosive device" the investigators discovered earlier, but said the crater found beneath the railroad bed was "1.5 meter by 1 meter in size."

Later Saturday morning, a second device went off on nearby tracks going the opposite direction, Vladimir Yakunin, head of Russian Railways, told Russian TV. He said no one was injured in the smaller explosion.

Markin said investigators are "studying the site of the accident, questioning the witnesses and conducting all kinds of forensic and technical examinations."

Federal Security Service Director Alexander Bortnikov said, "Criminology experts have come to a preliminary conclusion that there was an explosion of an improvised explosive device equivalent to seven kilos of TNT.

"Several leads are being pursued now. A criminal case has been opened under Article 205 ("terrorism") and Article 22 ("illegal possession or storage of weapons or explosives") of the Russian Criminal Code."

Russian Interior Minister Rashid Nurgaliyev said on TV that there are possible suspects in this crime.

"There are several people who could be involved in this crime," he said. One of them, he said, is a "stocky-built man of about 40 years old, with red hair."

"There are some traces left at the crime scene which could help in the investigation," he said. "We are getting a lot of information now, and I am very thankful for people who have responded to our requests to render their assistance in investigating this crime."

A total of 681 people -- 20 of them employees -- were on the Nevsky Express as it traveled from Moscow to St. Petersburg on Friday night. The Nevsky Express is Russia's fastest train, equivalent to a bullet train.

The crash happened at 9:25 p.m. (1825 GMT) when the train was 280 kilometers (174 miles) from St. Petersburg, Russian state radio said.

At least three carriages carrying more than 130 people derailed and turned on their sides, and emergency workers were working to free anyone who may still be trapped inside.

Yakunin told Russian TV that the company will pay a compensation of 500,000 rubles ($17,240) to the victims' families and 200,000 rubles ($6,897) to those injured.

The crash happened 44 minutes after another high-speed train, the Sapsan, had successfully traveled from Moscow to St. Petersburg on the same rails, a representative of the Russian Transport Police said during a video conference call Saturday.

In August 2007, an explosion on the tracks derailed the Nevsky Express, injuring 60 people in what authorities called a terrorist act. About 27,000 passengers on 60 trains were facing delays Saturday as a result of the accident, Russian State TV reported.

CNN's Maxim Tkachenko contributed to this report