COLUMBIA, S.C. — The U.S. Department of Justice is pressing federal regulators to come up with a way of keeping inmates from using cell phones in the nation’s prisons.

In a letter obtained Thursday by The Associated Press, Assistant Attorney General Beth Williams told the Federal Communications Commission that addressing the security threat posed by contraband cell phones “should be a chief priority” of both the FCC and Justice, which oversees the federal Bureau of Prisons.

The letter 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 cell signals of the phones, which are thrown over fences, smuggled by errant employees, even delivered by drone. South Carolina Gov. Henry McMaster also lobbied, writing Sessions an August memo on the dangers of prison cell phones and thanking him for any help he could provide.

A decades-old law says federal officials can grant permission to jam the public airwaves only to federal agencies, not state or local ones. Telecommunications companies are opposed, saying jamming cell signals could set a bad precedent and interfere with legal cell users nearby.