 The NKVD, the USSR's counterintelligence unit, first notified Mike Reilly, Roosevelt's chief of security, of the suspected assassination plot several days prior to Roosevelt's arrival in Tehran.Reilly had gone to Tehran several days early to evaluate security concerns and explore potential routes from Cairo to Tehran.Just before Reilly returned to Cairo, the NKVD informed him that dozens of Germans had been dropped into Tehran by parachute the day before.