The U.S. government does not have the authority to designate a domestic group a terrorist organization.
Under the current law, the State Department can designate foreign organizations as terrorist groups. But the U.S. has no domestic terrorism statute, and the FBI uses its territorial jurisdiction to prosecute domestic terrorism in the United States. Moreover, Antifa is not an organization with a leader, does not have a defined structure, or membership roles. Instead, Antifa is more of a movement of activists whose followers share a philosophy and tactics. Experts believe that the government can always investigate if they suspect terrorist activity, but it would raise concerns about privacy and the first amendment. Previously, the U.S government has come under severe scrutiny for closely shadowing the functioning of domestic organizations like the American Civil Liberties Union (ACLU).