Moscow, Russia (CNN) -- Russian officials say a terrorist explosion is the most likely cause of the derailment of an express train which killed at least 26 people and injured dozens more.
The head of the Russian Railroads company Vladimir Yakunin told state TV Saturday that investigators' "main theory behind the accident is the blast of an unidentified explosive device conducted by unidentified persons -- simply speaking, a terrorist act."
State TV also quoted survivors who said they heard a blast moments before the train derailed.
The Russian prosecutor's office is investigating whether the crash was caused by a terrorist attack, spokeswoman Marina Gridneva said. Police said they found a small crater beneath the tracks.
But authorities were also looking into the possibility there was a technical fault on the track.
Health Minister Tatyana Golikova said 96 people were injured, while the Russian prosecutor's office said 102 were hurt. Forty of the injured were in critical condition, Golikova said.
Eighteen people were still listed as missing after the Friday night crash, said Sergei Shoigu, the Russian minister for emergency situations.
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.
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.
Russian President Dmitry Medvedev and Shoigu of the emergency situations ministry also took part in the call. Shoigu said he would get more information as the derailed cars are lifted and authorities can see what is underneath.
In August 2007, an explosion on the tracks derailed the Nevsky Express, injuring 60 people in what authorities called a terrorist act.
Some 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