The official said the United States had warned Russia in recent weeks about the possibility of an attack. “We did warn the Russians appropriately,” said the official, speaking on condition of anonymity and without providing any additional details.

The number of casualties varies depending on the source. Official information released by the Federal Security Service (FSB) most recently spoke of 40 dead and more than 140 wounded, but the Baza news site said that the death toll is as high as 62.

The number of the dead can be expected to rise, sighting that a large portion of those wounded, about 60, were in critical condition.

The attack would be oe of the deadliest terror attacks in Russia since the 2004 Beslan school siege, when Islamist militants took more than 1,000 people hostage, including hundreds of children.

But the terrorists choice of where to strike is also bears some eerie similarities to the 2002 Moscow theatre hostage crisis. In that instance, a group of 40 Islamic Chechen fighters took more than 850 theatergoers hostage. The siege lasted four days in late October 2002, and ultimately resulted in the death of all the assailants and two officers, but most tragically, of 132 of the hostages, with some 700 being injured following the security forces ill-conceived attempt to incapacitate the attackers by pumping nerve gas into the theater.