Russia has apprehended eight suspects over the deadly explosion of the bridge linking annexed Crimea to Russia, the FSB security service said in a statement quoted by news agencies on Wednesday.
The suspects include five Russians and "three Ukrainian and Armenian citizens," it said, without providing more details.
"The explosives were hidden in 22 plastic film rolls weighing 22,770 kilograms (50,200 pounds)," it said.
The rolls left on a boat in August from the Ukrainian port of Odesa to Bulgaria. They then transited through the port of Poti in Georgia, then sent overland to Armenia before arriving by road in Russia, according to the FSB.
The explosives entered Russia on October 4 in a truck with Georgian license plates and reached the region of Krasnodar on October 6, two days before the blasts, the FSB said.
The "terrorist attack" was organized by Ukrainian secret services, with a Kyiv agent having coordinated the transit of the explosives, according to the FSB.
On Saturday, a blast ripped through the road and rail bridge connecting Crimea to Russia, killing three people, causing damage and igniting a massive fire.
The bridge is logistically crucial for Moscow – a vital transport link for moving military equipment to Russian soldiers fighting in Ukraine.
It is also hugely symbolic, with Russian President Vladimir Putin having personally inaugurated the structure in 2018.
The blast sparked celebrations from Ukrainians. Russia blamed the explosion on Kyiv on Sunday and on Monday launched missile attacks across Ukraine, killing at least 19 people and wounding more than 100.
On Monday, Putin said Russia's response to any further Ukrainian attacks would be "severe."
FSB claims foiled attack
In separate statements carried by Russian news agencies, the FSB also said it had foiled two attempted attacks allegedly planned by Kyiv on Russian territory: one near the capital Moscow and another in a city close to the border with Ukraine.
The FSB said it arrested a Ukrainian suspected of preparing to use a portable missile launcher for "sabotage and terrorist attacks" in the Moscow region.
It said the suspect, a citizen of Ukraine born in 1972, brought Igla missile launchers inside a secret compartment of a car, driving from Kyiv to Russia via Estonia.
The FSB said it had confiscated "means of communication" that confirm the suspect's contact with Ukraine's special services.
The FSB said it also detained another Ukrainian, born in 1967, accused of organizing an attack with explosives against a "transport-logistics hub" in Bryansk.
It said the suspect arrived in Russia via Estonia to one of the border regions of Russia, where he picked up a cache with components of explosive devices "prepared by Ukrainian special services".
The suspect then allegedly conducted reconnaissance at a warehouse of a transport and logistics company in order to plant an improvised explosive device and carry out a "high-power explosion", according to the FSB.
The FSB claimed both suspects have admitted their guilt and are cooperating with investigators.