U.S. Secretary of State Marco Rubio says Washington is taking steps to officially label the Muslim Brotherhood—a global Islamist movement that gave rise to Hamas—as a terrorist organization.

Speaking Tuesday on Sid and Friends in the Morning, Rubio explained that the process isn’t as simple as issuing a statement. Each branch of the Muslim Brotherhood operates differently, which means U.S. officials have to carefully review and build a case for every designation.

“All of that is in the works,” Rubio said. “There are different branches of the Muslim Brotherhood, so you’d have to designate each one of them.”

He acknowledged the U.S. government will almost certainly face legal challenges, but stressed the importance of having the evidence ready. “These things are going to be challenged in court,” Rubio noted. “You have to show your work—like a math problem—when you go before a judge.”

Rubio’s comments follow growing pressure from lawmakers in both the House and Senate to blacklist the Brotherhood. If the move goes through, the U.S. would join countries like Egypt, Saudi Arabia, the UAE, Bahrain, and Austria, which have already banned the group. Jordan outlawed it earlier this year as well.

Hamas, the Palestinian militant group, originated as an offshoot of the Muslim Brotherhood. Photo by Gage Skidmore from Peoria, AZ, United States of America, Wikimedia commons.