There are a lot of laws on the books that could be dusted off.
The residents of Mt Carmel, in Waco, TX, were a religious group that stayed pretty much to themselves, attacked nobody, and at worst violated a minor firearms law. Suddenly, the full force of the government came down on them in the form of armed assault from the FBI and ATF. They were called a dangerous cult by the government. As it turned out, the government was so fearful of these dangerous cultists, they went in with full military force, tanks and all, resulting in the immolation of almost all the residents and total destruction of their private property.
Why should a mosque, harboring those that wish to overthrow our government and kill all infidels be treated with any less distrust?