Within the Federal Bureau of Prisons, inmates are asked to “voluntarily” agree to electronic monitoring in order to use the bureau’s email system. Above, a prison cell block is seen at the Federal Correctional Institution, El Reno in Oklahoma in 2015.

A prison cell block is seen following a tour by US President Barack Obama at the El Reno Federal Correctional Institution in El Reno, Oklahoma, July 16, 2015. Obama is the first sitting US President to visit a federal prison, in a push to reform one of the most expensive and crowded prison systems in the world. AFP PHOTO / SAUL LOEB (Photo credit should read SAUL LOEB/AFP via Getty Images)