Khan, a U.S. citizen, is the sixth person in Texas to plead guilty to attempting to provide support for ISIS. Three others are awaiting trial: Kaan Sercan Damlarkaya in Houston, Said Azzam Mohammad Rahim in Dallas and Matin Azizi-Yarand in Plano, said Seamus Hughes, deputy director of the Program on Extremism at George Washington University and a national expert on homegrown extremist plots.