However, senior U.S. leadership knew these estimates of existing Soviet missile capabilities were completely inaccurate. Beginning with the collection of photo-intelligence by high-altitude U-2 overflights of the Soviet Union in 1956, the Eisenhower administration had increasingly-hard evidence that strategic weapons estimates favoring the Soviets were false. The CIA placed the number of ICBMs to be closer to a dozen. Continued sporadic flights failed to turn up any evidence of additional missiles. But the White House and the CIA wished to protect the secrecy of the source of the information--the photographs captured by the U-2 flying in illegal violation of Soviet airspace--and so they continued to hide the more accurate information that there were nearly zero Soviet ICBMs deployed. They kept the American public in the dark even though they knew from the start that the Soviets were monitoring the U-2 overflights. On the very day of the first U-2 overflight the Soviet ambassador to Washington protested the high-altitude violation of Soviet airspace, a fact denied by Washington and reported on by the press.