Russia's Federal Security Service (FSB) said on Wednesday that it had detained five Russians and three citizens of Ukraine and Armenia over the explosion that damaged the Crimea Bridge last Saturday.
The FSB said the explosion was organised Ukrainian military intelligence and its director, Kyrylo Budanov. The explosive device was moved from Ukraine to Russia via Bulgaria, Georgia and Armenia, the FSB said.
The FSB, which is the main successor to the Soviet-era KGB, also said that it had prevented Ukrainian attacks in both Moscow and the western Russian city of Bryansk.
Ukraine has not officially confirmed its involvement in the bridge blast, but some Ukrainian officials have celebrated the damage.
The explosion on the twelve mile-long bridge destroyed one section of the road bridge, temporarily halting road traffic. It also destroyed several fuel tankers on a train heading towards the annexed peninsula from neighbouring southern Russia.
The bridge, a prestige project personally opened by Russian President Vladimir Putin in 2018, had become logistically vital to his military campaign, with supplies to Russian troops fighting in south Ukraine channelled through it.
Russian forces launched mass missile strikes against Ukrainian cities, including power supplies. At a televised meeting of Russia's Security Council on Monday, Putin said the strikes were a retaliation for the Crimea bridge blast, which he said had been organised by Ukraine's secret services.