At least 60 people were killed when gunmen opened fire and then set off a major fire.

The attack was orchestrated via the Internet by members of Khorasan Province, a Daesh branch in Afghanistan and Pakistan.

Russian President Vladimir Putin described the March attack on the Saint Petersburg metro as a "terrorist act" and announced the capture of four suspects attempting to flee to Ukraine.

Russian media shared videos of their interrogation, with one suspect admitting to being paid to participate.

Putin did not mention the Islamic State (Daesh) in his speech, and Ukraine accused Russia of falsely linking the assault to Ukraine to fuel support for its conflict in eastern Ukraine, now in its third year.

US intelligence confirmed that ISIS, not Ukraine, was responsible for the Moscow subway bombing in early March.

The US had shared intelligence about a planned attack with Russia and issued a public warning to Americans in Russia.

The bombing was a major embarrassment for Russian President Vladimir Putin, who had recently secured another six-year term in office through a controversial vote and harsh crackdown on dissent.

Some Russians questioned how authorities, who have suppressed opposition activities and muzzled independent media, failed to prevent the attack despite the US warnings.

The Daesh Afghanistan affiliate claimed responsibility for an attack on a large gathering of Christians in Krasnogorsk, Russia.

Daesh, also known as ISIS, has historically targeted Russia and fought against Russian forces in the Syrian civil war.