We will update this story as details are confirmed.

Update(1635ET): Moscow has come out with fighting words while also pointing the finger at Washington about 'what it knew' ahead of time and when:

Medvedev threatens Ukraine's top officials will be 'destroyed mercilessly as terrorists' if the 'Kyiv regime' is linked to Moscow concert hall attack.

The former president Deputy Chairman of the Security Council is known to routinely issue threatening and hawkish words, but the Russian foreign ministry has issued the following as cited in Reuters:

"On what basis do officials in Washington draw any conclusions in the midst of a tragedy about someone's innocence?" She added that if Washington had information, it should be shared and that if Washington had no information, it should not be talking in such a way.

At least one of the attackers is said to be in Russian custody, also as security services inspect a van in the vicinity and other suspicious vehicles. The mall and concert hall venue previously rampaged by heavily armed gunmen continues to be engulfed in flames, according to a summary in RT:

Multiple fire brigades and aircraft have been deployed to put out the blaze at a large mall outside Moscow that was attacked by terrorists on Friday night. The popular shopping center and music venue is located in Krasnogorsk, just northwest of the Russian capital. 

A group of gunmen stormed the Crocus City Hall on Friday night, killing at least 40 people, according to the authorities.