The key suspect accused of plotting the St Petersburg metro bombing two weeks ago has been arrested near Moscow, BBC quoted Russian security officials as saying.
The FSB said Abror Azimov was detained in Moscow suburb of Odintsovo.
Fifteen people died and dozens were injured as a result of the April 3 attack, in which an explosion tore through a train traveling between stations in Russia's second-largest city.
The suspect has been taken for questioning to the Investigative Committee, which probes serious crimes, the FSB said.
Authorities identified the bomber as Akbarjon Djalilov, 22.
Both cities have large populations of migrant workers from the former Soviet republics of Central Asia.
He was carrying a pistol and a package containing two smartphones at the time of his arrest, reports said.
The FSB said the suspect was born in 1990 and trained the suicide bomber, according to TASS.
Russian Federation was once a hotspot for terror attacks, but the country has experienced relatively few in recent years. At least one of those arrested is believed to have been a recruiter for the "Islamic State" militant group. In 2002, Chechen rebels killed 170 hostages in a theater in the capital, Moscow.