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KYIV, Ukraine — Russia’s domestic intelligence service announced the arrest of eight people on Wednesday in connection with the weekend bombing of the bridge linking Russia to the occupied Crimean Peninsula. Five are citizens of Russia, according to the agency, the F.S.B., and the others are Ukrainian and Armenian.
President Vladimir V. Putin has blamed Ukraine for the blast, which he called a “terrorist attack,” and retaliated with a barrage of missile strikes against civilian targets in Ukraine this week, killing more than 20 people.
In a statement, the F.S.B. offered Russia’s first detailed version of how it contends the blast took place. It said that the bomb had contained 22 tons of explosives that were shipped out of a port in Odesa, in southern Ukraine, in August. The explosives made their way to southern Russia, where they were loaded onto a truck that was driven onto the bridge and detonated, it said.
The details could not be independently confirmed. Russia maintains an effective blockade on the ports of Odesa, permitting only grain ships, inspected by international monitors, to leave under a deal brokered this summer by the United Nations.
The F.S.B., as Russia’s premier domestic intelligence service, has primary responsibility for security on the bridge. The bombing represented a profound lapse in the agency’s oversight, even as Ukrainian officials telegraphed for months their intentions to strike the structure.
The F.S.B said Ukraine’s military intelligence service, the G.U.R., had masterminded the blast, saying that the agency’s commander, Gen. Kyrylo Budanov, bore personal responsibility. A senior Ukrainian official, speaking on the condition of anonymity because of a government prohibition against discussing the episode, confirmed that Ukraine’s intelligence services had carried it out, and other senior officials have not denied Ukraine’s role.
But Ukraine’s government has not officially claimed responsibility for the explosion, and a spokesman for the G.U.R. dismissed the Russian assertions as “nonsense.”
The F.S.B. and Russia’s main investigative committee “are fake structures serving the Putin regime, and so we will certainly not be commenting on their latest statement,” said Andrei Yusov, the G.U.R. spokesman.
The Crimea explosion, just after 6 a.m. on Saturday, was both a strategic and symbolic attack. The bridge is the sole link between Russia and Crimea, and a symbol for President Vladimir V. Putin of Russia of one of his greatest triumphs as leader: the illegal annexation of Crimea from Ukraine in 2014. The force of the blast sent a large chunk of the bridge tumbling into the sea and set fire to a train pulling fuel that was passing on a parallel railroad bridge.
The bridge is also the primary supply route for fuel and heavy equipment for Russia’s troops fighting in southern Ukraine. Any disruption to the structure would hinder Russian forces’ ability to fight at a time when Ukraine’s military is pushing deeper into territory taken by Russia at the start of the war.
The F.S.B.’s claims regarding the bomb’s size strain credulity, given the history of improvised explosive devices used in recent armed conflicts. During the American occupation of Iraq, the largest improvised bombs commonly made by insurgents were those placed in dump trucks, carrying approximately five tons of homemade explosives.
The most powerful non-nuclear air-dropped bomb used by the United States military — the GBU-43/B Massive Ordnance Air Blast, or MOAB — contains the equivalent power of just over nine tons of TNT. At 30 feet long and more than three feet in diameter, a MOAB can only be dropped from military cargo planes. The Pentagon has disclosed only a single use of the MOAB by U.S. forces in combat, during a 2017 attack on a suspected insurgent cave complex in Afghanistan.
Twenty-two tons is more than double the maximum capacity of most dump trucks. It appeared that Russia’s intelligence services were offering an estimate based on the loading limits for standardized 20-foot or 40-foot shipping containers, which are usually carried by tractor-trailers. In videos captured at the time of the blast, the truck that was detonated on the Kerch Strait Bridge did not appear to be carrying such a shipping container.
The F.S.B identified the driver of the truck as Makhir Yusubov, born in 1971. The senior Ukrainian official said it was likely that the truck’s driver had died in the blast, though it was not clear whether he was aware that the truck was carrying a bomb.
Shortly after the explosion, a man claiming to be Mr. Yusubov’s nephew gave an interview to a Russian news outlet claiming to be the owner of the truck, but denying that he had any knowledge about a plot to blow up the bridge.