Moscow, Russia (CNN) -- Eighteen people were still missing Sunday, according to Russian news agencies and state television, more than a day after a train derailment killed 26 people in what authorities call an "act of terror."
Meanwhile, rail services between Moscow and St. Petersburg were restored early on Sunday, Russian Railways company said.
Broken train cars and debris from the wreck lay on either side of the restored railway, but will be removed in the coming days, officials said.
Investigators say an improvised explosive device caused the express train to derail on Friday night, killing at least 26 people and injuring about 100 others.
"Elements of an explosive device" have been found at the site, authorities said, and the explosion made a small crater.
There are a number of factors that could explain the number of missing, an anonymous law enforcement source told the Interfax news agency.
There are some body parts that have yet to be identified, the source said, while other names could be people who bought a ticket but weren't on the train for some reason. In addition, a number of people were not injured and left the scene before rescue teams arrived, the source said.
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. Emergency workers freed people who were trapped inside.
Saturday morning, Russian Railways head Vladimir Yakunin told Russian TV, a second device went off in the area on the parallel track of the railway in the opposite direction. He said no one was injured in what was a smaller explosion than the first one, but it prompted the need for some repairs.
"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 wouldn't 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."
He 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 on Friday night 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."
There was no immediate word on who or what group might have been behind the action. But 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," he said.
"I would also like to say that we have (collected) a lot of material evidence that could give us leads to resolving the crime."
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 track, 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