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In page Lockheed U-2:

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Soviet radar monitored the U-2 incursion into Soviet airspace in real-time, with radar tracking starting from the time the aircraft crossed into East German airspace. Soviet leader Nikita Khrushchev was informed immediately.[citation needed]. While contemplating appropriate retaliatory steps, he ordered Soviet Ambassador to Washington, Georgy Zarubin, to protest vehemently to the U.S. State Department that very day, explaining that the recent trust-building to ease tensions between the two countries was undermined by the overflight provocations.[6]