 ABC's Jay O'Brien. We've learned that a law enforcement officer did confront would-be assassin Thomas Matthew Crooks, but was unable to stop him. There was apparently a desperate attempt to stop the shooter before he got those shots off. But the critical question is, how was the shooter able to get on that roof in the first place unimpeded? Right now, everything is being looked at in this extraordinarily dangerous threat environment. The president is also calling for an independent investigation into what took place