At least 133 people were killed and more than 145 were injured on Friday in an attack after a group of armed individuals opened fire at the Crocus City Hall music venue near Moscow, Russian state media agency TASS reported, citing the Russian Investigative Committee.

The number of reported casualties would make this one of the deadliest attacks near Russia's capital in recent years.

Here's what we know so far:

There are always questions after any attack as to why it was not stopped or detected. But the Moscow attack raises particularly difficult issues for Vladimir Putin at a time of international tension and mistrust. And much of that comes because of a warning from Washington.

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.

The official death toll from Russian authorities in the wake of Friday's terror attack on a Moscow concert hall has reached 133 killed, with some unofficial reports saying there could be as many as 140 or more. As the scene continues to be processed and investigated, and given some of the hospitalized are in critical condition, the death toll is likely to rise. Over 150 were wounded in the attack, which involved a group of well-equipped attackers sporadically shooting into crowds as they walked through the mall and into the concert hall.