A ninth person has been detained in connection with the St Petersburg metro bombing earlier this month.
Russian security officials say the suspect, Abror Azimov, hails from central Asia and is being questioned on suspicion of helping to prepare the deadly attack.
Fourteen people were killed and more than 50 wounded when Kyrgyz-born Russian Akbarzhon Jalilov detonated the device. He was killed in the explosion.
Russian news agency RIA cites FSB security officials as saying, Jalilov is believed to have been trained by Azimov.
At the time of his arrest, Azimov was reportedly carrying a pistol and a parcel containing two smartphones.
He was detained in the same Moscow suburb as two other suspects in the April 3 attack. The six others were intercepted in St Petersburg. All nine come from Central Asia.