The arrests this month in three U.S. cities of eight foreign nationals with suspected ties to terror group ISIS has further heightened concerns about a possible attack on American soil, officials are warning. Since the arrests, Sen. Lindsey Graham (R-S.C.) has requested a classified briefing for all senators detailing ISIS threats against the U.S. in a letter tp Majority Leader Chuck Schumer (D-N.Y.) and Minority Leader Mitch McConnell (R-Ky.). “I believe that the threat is urgent,” Graham wrote. One source told CNN: “ISIS isn’t done with us.” Former acting CIA director Michael Morell starkly …