The 7 March warning from the US to its own citizens was unusually specific. It talked of reports that "extremists" had "imminent plans to target large gatherings in Moscow" and specifically mentioned concerts. It advised Americans in the city to avoid large gatherings over the coming 48 hours.

As well as the public warning to its own citizens, the US also said it had communicated with the Russian government directly.

"The US government also shared this information with Russian authorities in accordance with its long-standing 'duty to warn' policy," a US official said in a statement after the attack.