Complex sentence: The KGB tried repeatedly to recruit him, but never was successful, and allegations that he had spied for the Soviets are unsupported, and in some instances, contradicted by voluminous KGB and Venona documentation released after the fall of the Soviet Union.
Split into simpler sentences: The KGB tried repeatedly to recruit him, but never was successful. Allegations that he had spied for the Soviets are unsupported, and in some instances, contradicted by voluminous KGB and Venona documentation released after the fall of the Soviet Union.