A bomb blast tore through a subway train deep under Russia's second-largest city St. Petersburg killing several people and wounding many more in a chaotic scene that left victims sprawled on a smoky platform.
Earlier this month, six people in St. Petersburg and two others in Moscow were detained for suspected involvement in the attack.
Azimov Abror Akhralovich, 27, a native of the Central Asian region, was detained near Moscow and brought to the Russian Investigative Committee, the service said in a statement.
Jalilov was killed in the explosion.
RIA reported that the FSB described him in a statement as one of the organisers of the attack and said he had trained Jalilov.More news: Doctor Prescribed Oxycodone for Prince in Another Name, Warrants Show
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At the time of his arrest, Azimov was reportedly carrying a pistol and a parcel containing two smartphones.
The bomber, Jalilov, developed an interest in Islam and went to Turkey, according to police, but they are unsure whether he then went to Syria, or whether he meant to also kill himself in the blast.
There has been no claim of responsibility for the attack but investigators say they are probing possible links to Islamic State jihadists.
Both cities have large populations of migrant workers from the former Soviet republics of Central Asia.