How were inmates communicating about alleged plot to attack power grid?

Brandon Russell and Sarah Beth Clendaniel are accused of plotting a sniper-style attack on several electrical substations in the Baltimore area. Russell founded a Florida-based neo-Nazi group and was on the FBI’s radar. In 2017, he was sentenced to five years in federal prison for having an unregistered destructive device and storing explosive material in his apartment. How were they able to communicate?