A Brooklyn man was arrested on Wednesday for providing assistance to ISIS in its efforts to recruit terrorists and attack Americans, according to Justice Department officials.

Zachary Clark, 40, was spotted by officials of the Joint Terrorist Task Force in pro-ISIS chat rooms encouraging others to commit atrocities against Americans on behalf of the terrorist organization. Clark laid out plans for others to execute in “well-populated locations” in order to maximize casualties.

“The defendant allegedly provided instructions for how to plan attacks on U.S. soil, encouraging ISIS supporters to attack in well-populated locations,” said Assistant Attorney General for National Security John Demers in a statement about Clark’s arrest.

U.S. Attorney Geoffrey Berman for the Southern District of New York explained that Clark had pledged his loyalty to ISIS once in July 2019. At the time, ISIS was led by Abu Bakr al Baghdadi. Baghdadi killed himself during a raid by U.S. forces in October. Clark then re-pledged his loyalty to the terrorist organization under the leadership of ISIS’s new leader, Abu Ibrahim al Sashemi al Qurayshi.

“As alleged, Zachary Clark twice pledged allegiance to ISIS, and posted on encrypted pro-ISIS chat rooms numerous exhortations and instructions on bomb-making and other terrorist acts to be carried out in New York,” Berman said. “ Thanks to the Joint Terrorist Task Force, Clark now faces serious criminal charges for his alleged support of a terrorist organization bent on killing Americans.”

FBI Assistant Director-in-Charge William Sweeney Jr. noted that Clark had “disseminated hate-filled messages via encrypted chat rooms, and encouraged like-minded individuals to carry out vicious attacks in the name of jihad.”

Clark had encouraged ISIS supporters to carry out several different types of attacks, including violence using bombs and knives. In one post about committing lone wolf knife attacks in the New York City subway system, Clark wrote that discomfort at “the thought of plunging a sharp object into another person’s flesh” is “never an excuse for abandoning jihad.”

Sweeney explained that the arrest was a success but noted there could be many others looking to do the same damage in the United States. Clark was charged with attempting to provide material support to ISIS and distributing information relating to explosives, destructive devices, and weapons of mass destruction, both of which carry sentences of up to 20 years in prison.