Complex sentence: They found John Lansdale, Jr., who had interrogated Oppenheimer over the Chevalier incident in 1943, and strongly supported him, particularly persuasive, and on August 11,1947, they unanimously voted to grant Oppenheimer a Q clearance.
Split into simpler sentences: They found John Lansdale, Jr., particularly persuasive; he had interrogated Oppenheimer over the Chevalier incident in 1943, and strongly supported him. On August 11,1947, they unanimously voted to grant Oppenheimer a Q clearance.