His father told law enforcement he assumed his son had gone to the shooting range at the The Clairton Sportsmen's Club to practice with his rifle and would be back by 1pm on Saturday,
CNN reported.
But as time went on he grew more concerned and phoned local law enforcement.
At around 3pm on Saturday, Crooks was spotted at the rally's security screening area and tried to pass through metal detectors with the rangefinder, which is used by shooters to measure distance.
The device, which looks like a small pair of binoculars, would not have prevented him getting into the rally in Butler,
Pennsylvania, but did put him on law enforcement's radar.
The Secret Service watched him but lost track of Crooks when he left the secure area.
He is then believed to have collected the rifle he used to shoot Trump, kill a rally attendee and wound others from his car, which also contained a bulletproof vest and two explosive devices.
Then, about 40 minutes before the shooting, Crooks was back on the Secret Service radar.
A photograph showing the sniper appearing to crawl on the ground while scouring the area was circulated to law enforcement as a suspicious sighting at 5:30pm.
Around 6:11pm, Crooks took up a position on a slanted roof and shot at Trump from a distance of around 165 yards, wounding him in the ear and killing retired fire chief Mr Comperatore.
A Secret Service counter-sniper team codenamed 'Hawkeye' also spotted him looking at their position through the rangefinder.
The latest bombshell developments raised more questions about the massive security failures that led to the attempted assassination of the former president.

The remote detonator is seen next to Thomas Crooks' cellphone
The gunman was also
acting strangely around the metal detectors that attendees were forced to walk through before entering the event grounds. Witnesses said that Crooks was furiously checking his phone and operating a hunting range finder.
He attempted entry around this point but set the detector off and was refused, sources said.
At around 3pm on Saturday, Crooks was spotted at the security screening area and tried to pass through magnetometers with device used to measure distances.
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