Thomas Matthew Crooks hid rifle in advance of Trump rally sniper attack, Secret Service source says

Two FBI investigators scan the roof of AGR International Inc, the building adjacent to the Butler Fairgrounds, from which alleged shooter Matthew Thomas Crooks fired at former President Trump in Butler, Pennsylvania. (Jeff Swensen/Getty Images)

Thomas Matthew Crooks, the 20-year-old would-be assassin who opened fire on former President Donald Trump's Pennsylvania campaign rally Saturday, hid the weapon in advance, according to a Secret Service source.

It was not immediately clear where he hid it, however. By the time agents spotted him on the roof, he was already holding it.

"We went from golf range finder to AR-15, and now we have to fill in the gap," the source told Fox News.

When authorities first observed Crooks carrying a golf range finder Saturday, he was perceived as a "person of interest" but not a "threat," authorities said Thursday.

Range finders were not banned from rally events at the time, but authorities are expected to review the list of items that are not allowed.

He did not become an official threat until he was seen with a weapon.