 Law enforcement sources telling ABC News the Secret Service sniper faced a harrowing split-second decision. Once the sniper noticed the suspect was on the rooftop, he had to determine whether the man posed a threat. And when the shots were fired, it became apparent the sniper opened fire, killing the suspect. The security breach all the more maddening because senior federal law enforcement officials have been warning for months that the threat environment is perhaps as dangerous as it was before 9-11.