Stunning new details from @CBSNews
 on how secret service snipers noticed the shooter before the Trump rally in Butler, PA on Saturday.

There were three snipers stationed INSIDE the building the shooter was on, according to a local law enforcement officer with direct knowledge.

One of the snipers inside saw Crooks outside looking up at the roof observing the building and then disappeared, according to local law enforcement. 

Then Crooks came back, sat down and was looking on his phone and at that point one of the snipers took a picture of him.

Then Crooks took out a range finder and the sniper radio-ed to the command post.  Crooks disappeared again and then came back a third time with a backpack. The snipers called in with information that he had a backpack and said he was walking towards the back of the building.

Officers believe that Crooks might have used an air conditioning unit to get on top of the roof.

By the time other officers came for backup he had climbed on top of the building and was positioned above and behind the snipers inside the building.

Two other officers who heard the sniper’s call came and tried to get onto the roof.

State police started rushing to the scene. But that time a secret service sniper had already killed Crooks, according to the law enforcement officer.

The officer said also Crooks had an IED in his pocket in addition to the explosive materials in his car.

Questions remain about HOW this was possible that several law enforcement officers had eyes on the shooter before and reported his behavior... yet this still happened.