Why did the Soviet leader Nikita Khrushchev demand an apology from President Eisenhower in 1960?

Because the U.S. sent the U-2 spy plane over Russia to...well...spy. The plane was later shot down by the soviets and the pilot was captured, they knew it was an American plane because

1. they had the pilot

2. American flags where one the plane

President Eisenhower never gave an apology.