what does " should have thrown up a security screen" mean? The Secret Service should have thrown up a security screen. But the disaster had exposed a hidden weakness—the allegiance of individual agents to a man, not an office. As long as Kennedy had been in command the lines of authority were clear. Now the old order had been transformed into hopeless disorder. Theoretically Roy Kellerman was still the agent in charge. Emory Roberts had already defied him, however, and when Roy issued instructions that all the agents who had been riding in Halfback were to guard the hospital's entrances, nobody bothered to point out that Roberts had undercut him by reassigning them.