People gather in central Moscow on April 6, 2017, to pay tribute to the victims of the April 3 St. Petersburg metro blast. (Maxim Shemetov/Reuters)

Russian investigators said Thursday that they have arrested eight possible accomplices in the St. Petersburg subway attack Monday that killed 14 people and wounded dozens more, and a Russian news agency reported that the suspected bomber may have trained with the Islamic State in Syria.

The explosion in the heart of Russia’s cultural capital shocked the nation and brought outpourings of grief, followed by three days of official mourning. The first subway bombing in seven years also pierced a sense of security in Russia, and authorities moved Thursday to ease those fears.

As thousands of Russians attended carefully staged rallies in major cities across the country, state television broadcast dramatic footage of police breaking into a St. Petersburg apartment and bringing out three men in handcuffs.

The Investigative Committee, a federal authority with broad powers, said six people were detained in St. Petersburg and two in Moscow. Investigators also turned up explosives identical to those police discovered in another St. Petersburg station soon after the subway attack.

The arrests are considered a possible breakthrough in the investigation of what led Akbarzhon Dzhalilov, a 22-year-old ethnic ­Uzbek who came to Russia from a volatile area of Kyrgyzstan, allegedly to detonate a bomb on a train as it traveled between two central St. Petersburg subway stations. Investigators so far have not been able to determine whether Dzhalilov acted alone or with accomplices. No group has asserted responsibility for the attack.

(The Washington Post)

[St. Petersburg bombing sets a city on edge]

Kyrgyzstan is one of the impoverished, majority-Muslim Central Asian countries that sprang from the ruins of the Soviet Union and have become breeding grounds for Islamist fundamentalism. The Islamic State is believed to have recruited thousands of residents of these countries to join the militant movement in its strongholds in Iraq and Syria.

The Fontanka.ru agency said Dzhalilov had traveled to Syria in 2014 and trained with Islamic State militants. The report said that Russian investigators were trying to determine his travels but that they had ascertained that the device used in the subway attack bore the hallmarks of “Syrian know-how,” specifically traces of burned sugar.

In Moscow, at least 10,000 people gathered in a square outside the red-brick Kremlin wall for a hastily organized rally titled “Piter, we’re with you!” — using Russian slang for St. Petersburg. Participants streamed into the square to the beat of pop music, many carrying red carnations.

There had been suspicions among Muscovites about the motives of the organizers of the event, especially after an item — since deleted — appeared on the site massovki.net offering $7 to anyone willing to sign up to attend the gathering. The site is often used to attract crowds to television shows, concerts and pro-Kremlin rallies.

Some analysts suspected that authorities had organized the demonstrations and had urged people to attend in order to show that they could get at least as many people out in the street as opposition leader Alexei Navalny did March 26, when tens of thousands across Russia rallied against official corruption.

“This is an indirect response to the mass rallies against corruption, the large scale of which came as a surprise to the authorities,” Alexei Makarkin, an analyst at the Center for Political Technologies, told the Vedomosti newspaper. Thursday’s anti-terrorism rallies, he said, were intended to “demonstrate that the opposition rallies were small in number.”

1 of 18 Full Screen Autoplay Close Skip Ad × The scene in St. Petersburg after a subway blast killed at least 9 people View Photos An explosion tore apart a train as it traveled between two central St. Petersburg metro stations. Caption An explosion tore apart a train as it traveled between two central St. Petersburg metro stations. April 3, 2017 Men carry an injured person on a stretcher outside a metro station. Alexander Tarasenkov/AFP/Getty Images Buy Photo Wait 1 second to continue.

[ Russia’s mass protests were a win for Navalny. How will the Kremlin respond? ]

Sergei Dorenko, editor of the Govorit Moscow radio station, encouraged his listeners to attend the Moscow event, even though he said the countrywide rallies “remind us of the strange Soviet tradition” of mandatory participation in state demonstrations.

“I, too, find that annoying, disgusting and unpleasant,” Dorenko said. But “everyone wants to express their solidarity with [the people of St. Petersburg] and say ‘No’ to terror.”

The government has taken pains to deny that the rally was political in nature. A spokesman for Russian President Vladi­mir Putin denied government involvement, and several times during a national live broadcast of the ­Moscow rally on state-run television, the announcer asserted that the crowds had “come here voluntarily.”

But some people interviewed Thursday suggested that their presence was slightly less than voluntary. One of them, who gave his name as Sylvester Mikhalyov, said he was attending with nearly 100 fellow students of a Moscow university. He said administrators had handed out the carnations and told the students to attend.

“It’s a terrible thing, the loss of life. Of course I’m against it, so it’s not like I was against coming,” he said.

Others said they had shown up without any official encouragement. Leaning against the brick facade of the State Historical Museum, Lyudmila Makarova, 58, said she and some friends decided to attend the rally after hearing about it on television. She said she called several relatives in St. Petersburg as soon as she found out about the attack, and then cried. The violence still frightened her, she said, and she repeatedly referred to the attack as a “horror.”

Although many of the protesters carried flags of political parties — the Communists and the ultranationalist Liberal Democratic Party of Russia were well represented, as was the pro-Kremlin National Liberation Movement — an organizer asked them to lower their banners because “today is not the day for political ambitions.”

A protester who gave only his first name, Vitaly, briefly held aloft a sign proclaiming “Corruption Kills!” It was an echo of the anti-corruption rallies last month, but also, he said, was relevant to the terrorist attack. He told a correspondent that he believed terrorists in Russia were able to evade capture because police are corrupt.

Moments later, two police officers in neon-green vests approached and told him to lower the sign.

Natalya Abbakumova in Moscow contributed to this report.

Read more:

Mourners gather for the young victims of St. Petersburg

Ethic violence spreads in Kyrgyzstan

Today’s coverage from Post correspondents around the world

Like Washington Post World on Facebook and stay updated on foreign news