A surge of confidence by supporters of the Islamic State terror group — reflected in a series of online threats against Europe combined with its deadly attack on a concert hall in Russia — is giving security officials in the United States cause for concern.

National security and law enforcement officials have long been worried about small groups or individuals drawing inspiration from terror plots around the world to conduct attacks in the U.S. But FBI Director Christopher Wray told lawmakers Thursday that something more worrisome may be in the works.

“Now increasingly concerning is the potential for a coordinated attack here in the homeland, akin to the ISIS-K attack we saw at the Russia concert hall a couple weeks ago,” Wray warned lawmakers, using an acronym for the Islamic State’s Afghan affiliate.