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... Charged with hunting down Ukrainian saboteurs inside Russia, with keeping anti-Kremlin activists in check, and with disrupting the operations of hostile foreign intelligence agencies, the FSB, the main successor agency to the Soviet-era KGB, has its hands full.

That, say former U.S. intelligence officials and Western security analysts, helps explain why it may have overlooked other threats, including that posed by Islamist militants, such as ISIS-K, which claimed responsibility for the attack.

"You can't do everything," Daniel Hoffman, a former senior CIA operations officer who served as the agency's Moscow station chief, told Reuters.

"You dial up the pressure on the locals and sometimes you don't get the intelligence you need on a potential terrorist attack. That's where they failed.

"It's possible they're overextended dealing with the war in Ukraine and dealing with political opposition. This one slipped through the cracks." ...