BETHEL PARK, Pa. − A local police officer climbed a ladder to the roof of the building where he encountered a gunman moments before the start of the deadly shooting at former President Donald Trump's campaign rally in Butler, Pennsylvania, a local law enforcement official said.

Butler County Sheriff Michael Slupe told the Associated Press the gunman, later identified as Thomas Matthew Crooks, saw the officer and turned toward him just before the officer climbed back down to safety. Slupe, who said the officer was unable to use his own gun under the circumstances, said Crooks quickly opened fire toward Trump, who was speaking in the Butler Farm Show grounds about 150 yards away on Saturday.

Secret Service snipers then fatally shot Crooks, the agency's director has said. The Secret Service issued a statement Tuesday saying the agency was "deeply grateful" to local police officers for their efforts − and denied blaming local authorities for the attack.

Trump was wounded, one attendee was killed, and two others were critically wounded. Secret Service Director Kimberly Cheatle, in an interview Monday on ABC News, said local police had been in the building and were "responsible for the outer perimeter of the building." The statement drew criticism from Patrick Yoes, national president of the Fraternal order of Police, who said all officers at the scene acted "heroically."

"This is a failure at the management or command level who failed to secure an obvious weakness in the security of this event," Yoes said in a statement.

The Secret Service, in a statement early Tuesday morning, appeared to walk back the blame.

"We are deeply grateful to the officers who ran toward danger to locate the gunman and to all our local partners for their unwavering commitment," the agency's statement read. "Any news suggesting the Secret Service is blaming local law enforcement for Saturday's incident is simply not true."

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Developments:

∎ Three snipers from local tactical teams, deployed to assist the U.S. Secret Service, were stationed inside the building from which Crooks opened fire, a local law enforcement officer with direct knowledge of the incident told CBS News.

∎ Video posted from the rally to social media shows civilians calling out to police that someone was on the roof moments before the shooting started. Cheatle said "a very short period of time" passed between then and when Crooks began shooting.

∎ The Republican National Convention entered its second day Tuesday, one day after Trump made his first public appearance since the shooting.

Crooks reportedly bought ladder before shooting

Hours before the shooting, Crooks stopped at a Home Depot in his hometown of Bethel Park and purchased a ladder, CNN and NBC reported, citing unnamed law enforcement officials. Several rallygoers reported to local officers before the shooting that Crooks was acting suspiciously and was climbing a ladder, according to a law enforcement official who spoke to AP on the condition of anonymity because they were not authorized to discuss the investigation.

In a statement shared with USA TODAY, Home Depot deferred to law enforcement but condemned the shooting.“The best source of information on this is law enforcement. We condemn the violence against former President Trump, and our thoughts are with him, the other victims of Saturday's horrific events, and their families,” the statement said.

Neighbor of shooter stunned by tragedy

Officers with the Bethel Park Police Department on Tuesday continued blocking access to the street where the Crooks family home sits. The FBI has said the family has been cooperating with the investigation. Late Monday night, someone stepped out of the house, got into a red SUV parked in the driveway and left as photographers hurried to capture a photo. Some neighbors who live within a block of the Crooks say they’re still in disbelief that one of their neighbors tried to assassinate Trump. Vanessa Ovesney, 17, said it was a surprise to see the shooter was someone who had grown up in the peaceful suburb about an hour from where the shooting took place. She said Crooks looked familiar but that she couldn’t recall any interactions with him, though she has passed his house countless times.“It’s unfortunate because we don’t know what was going on with him exactly,” she said.

Memorial Thursday for shooting victim Corey Comperatore

Friends and loved ones of the former volunteer firefighter who was fatally shot at Trump's rally can pay their respects at a memorial Thursday, the family of Corey Comperatore announced.

Comperatore, 50, was a "hero" who dived to protect his wife and daughter when the shooting started, Gov. Josh Shapiro said. An obituary released Monday says the father of two daughters was an engineer, 10-year veteran of the U.S. Army Reserves and a "man of God."

"Above all, Corey was the quintessential family man and the best girl dad," the obituary says, adding that "Corey's legacy is not just in the major milestones he achieved but in the small acts of kindness that marked his everyday life."

− Chris Kenning

The hunt for a motive continues

Authorities have yet to reveal a motive for the shooting. Despite scouring Crooks’ phone and computer search activity, authorities have been unable to determine why Crooks, a 20-year-old dietary aid at a nursing home with no criminal record, acted as he did. Interviews with family and friends have turned up no obvious motive, law enforcement sources told CNN.

Crooks would have been eligible to cast his first presidential vote in the Nov. 5 election. Public records reflect a mixed political background, as Crooks was a registered Republican who a few years ago made a $15 donation to a Democratic Party cause. Records also show his father is a registered Republican and his mother a registered Democrat.

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Authorites fear violence could breed violence

The Department of Homeland Security and Federal Bureau of Investigation remain "concerned about the potential for follow-on or retaliatory acts of violence" following the shooting, Politico reported Monday. The media outlet cited an internal agency bulletin it has viewed. The report added that "some online communities have threatened, encouraged or referenced acts of violence in response" to the shooting at Trump's rally.

The four-page bulletin mentions no specific targets, but it notes that extremists have conducted or plotted attacks against “perceived political or ideological opponents” in the past.

Contributing: Reuters