COLUMBIA, S.C. (AP) — The U.S. Department of Justice is pressing federal regulators to come up with a solution to combat cellphones in the nation’s prisons.

In a letter obtained Thursday by The Associated Press, Assistant Attorney General Beth Williams wrote to the Federal Communications Commission that addressing the security threat posed by contraband cellphones “should be a chief priority” of both Justice and the FCC. It follows an appeal from South Carolina’s prisons director to Attorney General Jeff Sessions in June, beseeching the top prosecutor for help pursuing FCC permission to jam prison cell signals.

Williams says smuggled cellphones enable inmates to continue criminal efforts outside prison and potentially plan insurrections. Telecommunications companies are opposed, saying jamming cell signals could set a bad precedent and interfere with legal cell users nearby.