COLUMBIA, S.C. (AP) — More than 50 members of Congress are giving federal regulators a deadline for resolving how to prevent prisoners from using contraband cellphones behind bars.

In a letter dated October 4 and obtained Thursday by The Associated Press, 48 House members and four U.S. senators wrote to the Federal Communications Commission, calling the security threat posed by contraband cellphones "an issue of critical importance."

Inmates use the phones, the members of Congress wrote, to continue the violent crimes and gang activity that landed them behind bars in the first place. The signe...