Eight out of 12 suspects were taken into custody over the attack on the Crimea bridge, the Russian security service said on Wednesday.
Kyrylo Budanov, the Ukrainian chief of military intelligence, is identified as the mastermind of the incident, the Federal Security Service reported.
The explosives used in the attack were sent from Odesa through Bulgaria, Georgia, and Armenia, and hidden in rolls of camera, Russia's Federal Security Service noted.
On Sunday, Russian President Vladimir Putin accused the Ukrainian intelligence of carrying out what he said was a "terrorist attack" on the Kerch Bridge.
A huge explosion early Saturday hit the Kerch Bridge, a key supply route for Russia which was built after it annexed Crimea in 2014.