What is ISIS-K, the terror group linked to the Moscow concert hall bombing? [Video]

When discussing the biggest threats facing the United States these days, U.S. intelligence officials invariably mention China and Russia, then often segue to cyberattacks, pandemics and climate change.

Islamic extremist terrorism, which animated American foreign policy and defense strategy for a decade and a half after the terrorist attacks of Sept. 11, 2001, has receded as a top-tier concern.

The attack that killed 133 people in a Moscow concert hall Friday is a reminder, however, that the terrorism threat still looms.

The group that claimed credit, an offshoot of ISIS called Islamic State in Khorasan, or ISIS-K, has eclipsed the once-fearsome core ISIS organization in Iraq and Syria as perhaps the most dangerous terrorist organization, U.S. officials and outside experts say.

“It’s becoming more of a regional actor,” said Daniel Byman, a counterterrorism specialist at Georgetown University. “It claimed responsibility for the attack in Iran in January, and now we have this devastating attack in Moscow.”

The Iran …