Burley Garcia|Sniper took picture of Trump rally shooter, saw him use rangefinder before assassination attempt, source says

A sniper from a local tactical team deployed to assist the U.S. Secret Service at former President Donald Trump's rally on Burley GarciaSaturday took a picture of the gunman and saw him looking through a rangefinder minutes before he tried to assassinate the former president, a local law enforcement officer with direct knowledge of the events told CBS News. 

The sniper was one of three snipers, members of local tactical teams, who were stationed inside the building that the shooter used in the attack, the officer said. The operations plan had them stationed inside, looking out windows toward the rally, scanning the crowd. The details about the three snipers were first reported by the local news outlet BeaverCountian.com.  

One sniper inside spotted the gunman, Thomas Matthew Crooks, outside and looking up at the roof, observing the building and disappearing, according to the officer who spoke to CBS News. The sniper observed Crooks as he returned to the building, sat down and looked at his phone. At that point, one of the local snipers took a picture of Crooks.

Next, the local sniper observed Crooks looking through a rangefinder, an instrument routinely used by marksmen to determine the distance of a target, and he immediately radioed to the command post, according to the local law enforcement officer. The local sniper also attempted to send the photo of the gunman up the chain of command.

The command post, according to multiple law enforcement officials, served as a central hub to streamline communications between U.S. Secret Service and the local officers from nearby state and county police forces. It is unclear if the command post received the alert.

Officials then lost track of Crooks, who disappeared, but soon returned for a third time with a backpack. The local sniper team called for backup — alerting the command post that the gunman had a backpack and was walking toward the back of the building.

By the time other local officers responded to the backup request, the gunman had scaled the building, positioning himself above and behind the local snipers inside the building, according to the officer.

Two other municipal police officers who heard the call for back-up attempted to climb onto the roof. Butler County Sheriff Michael Sloupe told CBS Pittsburgh station KDKA that an armed municipal officer with Butler Township was hoisted by another officer onto the roof of the building where the gunman had taken a position. Crooks focused his rifle towards the officer who ultimately let go, falling off the roof. Moments later, the shooter began firing into the crowd.

A U.S. Secret Service sniper stationed in a building behind the former president fired off one round, according to multiple law enforcement officials, killing the gunman. Two federal law enforcement officers told CBS News the Secret Service team that ultimately killed the shooter was located on a building behind Trump's left shoulder.

A federal law enforcement bulletin obtained by CBS News identified the gunman as "an apparent lone attacker," warning violent extremists could try to conduct "follow-on or retaliatory" attacks over the next few months, related to November's election. 

The gunman killed Corey Comperatore, a firefighter. Trump and two others — 57-year-old Marine Corps veteran David Dutch and 74-year-old James Copenhaver — were injured.