From BBC:
"It is too early to know who was responsible for this attack.
But it is notable that two weeks ago, the US issued an unusual warning to its citizens in Russia. It was "monitoring reports that extremists have imminent plans to target large gatherings in Moscow, to include concerts”.
The message said US citizens should avoid large gatherings over the next 48 hours.
Despite the delay, the similarity in the nature of the target suggests a link might be possible. But there was no indication of who those extremists might be.
The White House has quickly said there is no indication that Ukraine or any Ukrainians were involved in the attack."
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