What Matthew Thomas Crooks did in hours leading up to attack on former President Trump

In the days since the assassination attempt on former President Donald Trump, the pieces have begun to come together about what the shooter was doing in the lead-up to the attack.

The day before the shooting, Thomas Matthew Crooks went to a local shooting club and practiced on the rifle range, counsel for the Clairton Sportsmen's Club confirmed to ABC News.

The 20-year-old purchased ammunition shortly before the incident, both online and in-person, according to law enforcement sources.

He asked his employer -- a nursing home where he worked as a dietary aide -- to take the day off on the day of the shooting, but said he would be back as soon as the next day, sources said.

On the day of the assassination attempt, he asked if he could borrow his father's AR-15-style rifle, saying he planned to go to a shooting range, sources said. Because Crooks' father had previously let him use the gun for target shooting, he was not suspicious that his son was planning anything of concern.

Crooks went to a local gun shop that day, where he bought 50 rounds of 5.56 mm ammo. He also stopped at a Home Depot to buy a 5-foot ladder, though it was not found at the scene and is not believed to have been used to access the roof.

He then drove to the Trump rally in Butler, Pennsylvania, parking about a third of a mile away at a gas station.

For at least an hour before the shooting, Crooks was seen walking around near the rally area. Crooks was seen in video obtained by Pittsburgh ABC affiliate WTAE pacing just beyond the boundaries of the Trump rally at 5:06 p.m., around one hour and five minutes before shots were first fired.

He was also seen walking around near the magnetometers minutes before the shooting, according to law enforcement sources. Some witnesses reported him as a suspicious person, law enforcement sources said.

He then made his way to a nearby building, climbing up air conditioner units until he was able to get on the roof of an adjacent building, according to sources. From there, he moved onto a taller building next to it.

Just before Crooks opened fire, rallygoers spotted him on the roof and tried to alert the Secret Service.

In an unclassified briefing Wednesday, law enforcement officials said 20 minutes passed between when Secret Service snipers first spotted Crooks on the roof and when he opened fire on Trump. He was identified as a person of interest 62 minutes before the shooting, they said.

A Butler Township police officer attempted to stop the shooter, climbing up the roof to approach him, but Crooks pointed his gun at the officer, prompting the officer to retreat, according to Butler County Sheriff Michael T. Slupe.

It was at this point that Crooks began firing his weapon toward Trump, who at that moment turned to look at a Jumbotron, possibly saving his life.

A Secret Service sniper shot back at Crooks, fatally shooting him in the head 26 seconds later, according to law enforcement officials.