Russian security agents have detained a ninth person in connection with the St. Petersburg subway train bombing that killed 15 people and injured 45 people on April 3.
Abror Azimov, who was born in 1990 and is from Central Asia, was held in Odintsovo district west of the capital.
Russian news agency RIA cites FSB security officials as saying, Jalilov is believed to have been trained by Azimov.
Two suspects in the bombing have already been detained in the same Moscow suburb where the latest arrest was made.More news: Facebook murder suspect could be anywhere
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.
Earlier this month, six people from Central Asia were detained in St Petersburg on suspicion of recruiting for Daesh and aiding "terrorist activity".
There has been no claim of responsibility for the attack but investigators say they are probing possible links to Islamic State jihadists.
Jalilov is said to have detonated his device after a metro train left St Petersburg's Sennaya Ploshchad station, heading south for Tekhnologichesky Institut on the blue, north-south metro line.