Fourteen people died and dozens were injured in the April 3 blast which has been blamed on 22-year-old Akbarjon Djalilov, thought to be a Russian national born in Kyrgyzstan, who died in the blast.
Russian authorities detained one of the people suspected of organizing the St. Petersburg metro terrorist attack two weeks ago, TASS, the Russian government-owned news agency, reported Monday.
He has been identified as Abror Azimov, who was born in 1990 and is from Central Asia.
The FSB statement said Akhralovich was a "native of the Central Asia region" and allegedly had "trained the suicide attacker".More news: North Korea Displays Apparently New Missiles amid Rising Tensions across the Region
Russia's investigative committee said this decision may have helped prevent further losses. At least one of those arrested is believed to have been a recruiter for the "Islamic State" militant group.
Numerous previous attacks were aimed at progressing separatist causes in Russia's North Caucasus, including movements in Chechnya and Dagestan.
Azimov has been accused of training Jalilov to use the bomb, which he took with him on a train and detonated, killing himself in the explosion.
Seven other ethnic Central Asians have been held by police in both St Petersburg and Moscow.