According to Soviet reports, German agents planned to kill the Big Three leaders at the Tehran Conference, but called off the assassination while it was still in the planning stage. 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. The NKVD suspected German agents were planning to kill the Big Three leaders at the Tehran Conference.