Yuri Gagarin
In his 2004 book Two Sides of the Moon, Alexey Leonov tells that he was flying a helicopter in the same area that day when he heard "two loud booms in the distance." His thought was that the Sukhoi jet was flying below its minimum allowed altitude, and "without realizing it because of the terrible weather conditions, he passed within 10 or 20 meters of Yuri and Seregin's plane while breaking the sound barrier." The resulting turbulence would have sent the MiG into an uncontrolled spin. Leonov thinks the first boom he heard was that of the jet breaking the sound barrier, and the second was Gagarin's plane crashing.