Local police warned of suspicious person at Trump rally before shooting

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Local police in Butler, Pennsylvania, notified former President Trump's Secret Service detail during his campaign rally on Saturday before the assassination attempt that they were looking for a suspicious person in the area, a senior federal law enforcement source confirms to Fox News.

The federal law enforcement source told Fox News "there were multiple radio transmissions describing a search for a suspicious person after citizens reported this to local police. Local law enforcement leader confronted the individual at the rooftop."

The suspicious person, later identified as gunman Thomas Crooks, 20, was first alerted to local police by rally attendees on their way to the rally. The spectators said they observed Crooks pacing and acting strange near the metal detectors, according to NBC News . Local police began pursuing Crooks on foot and notified the Secret Service that they were searching for a suspicious person near the event.

It is unclear exactly when the Secret Service was flagged and whether it was before Trump took the stage at his rally. It was also not clear how long Crooks was on the roof, but sources told Fox News he was initially seen without the gun about 30 minutes before the attack.

A local officer noticed a suspicious man carrying a range-finder "in or just-outside" the venue before Trump took the stage at his rally, a law enforcement source told Fox News . The officer reported the sighting to state police and took a photo. There was a discussion about whether the item he was carrying was a pair of binoculars to try and see the rally better.

A few minutes into Trump's remarks, Crooks opened fire, attempting to assassinate Trump.