BUTLER, Pa. — Signs of trouble were evident in the minutes before shots rang out at Donald Trump’s rally in Pennsylvania: Police had a report of a suspicious man pacing near the magnetometers and were apparently exchanging photos of the suspect.

Witnesses pointed and shouted at an armed man on a nearby roof.

When a police officer climbed up to the roof to investigate, the gunman turned and pointed his rifle at him. But the officer did not — or could not — fire a single shot.

A sniper cut down 20-year-old Thomas Matthew Crooks within seconds of him firing an AR-style rifle toward the former president, but it was too late. Now investigators are trying to painstakingly piece together how an armed man with no military background managed to reach high ground and get the jump on teams of Secret Service agents.

President Joe Biden has ordered an independent investigation of the attempted assassination. Homeland Security Secretary Alejandro Mayorkas said he has “full confidence” in the Secret Service’s leadership, but he conceded that the gunman never should have reached that deadly position.

“We are speaking of a failure,” Mayorkas told CNN. “We are going to analyze through an independent review how that occurred, why it occurred, and make recommendations and findings to make sure it doesn’t happen again.”

In addition, the first congressional hearing about the shooting has been scheduled for next Monday, with the director of the U.S. Secret Service, Kimberly Cheatle. She’ll testify before the House Committee on Oversight and Accountability.

“The United States Secret Service has a no-fail mission, yet it failed on Saturday when a madman attempted to assassinate President Trump, killed an innocent victim, and harmed others,” said Rep. James Comer, the committee’s Republican chairman.

The leaders of the Senate Homeland Security and Government Affairs Committee also announced Monday they will conduct an investigation.

Sen. Gary Peters, the Democratic chairman of the committee, and Rand Paul, the ranking Republican,, said the briefing for members should happen before July 25, while a hearing will be sought as soon as possible, but no later than August 1.

“This committee has an obligation to unearth the truth about the failures on Saturday and before, no matter how inconvenient to the government,” Paul said. “We will leave no stone unturned.”

At least a dozen police officers and sheriff’s deputies were assisting the Secret Service and Pennsylvania State Police with rally security.

Several rallygoers reported to local officers that Crooks was acting suspiciously and pacing near the magnetometers, according to a law enforcement official who spoke on the condition of anonymity because the official was not authorized to discuss the investigation.

It was a Butler Township police officer who encountered the gunman on the roof before the shooting. The officer was looking for the suspicious person when another officer hoisted him up so he could grab the edge of the roof, local officials said.

The officer dropped back down to safety when the gunman turned and pointed his rifle at him, according to Butler County Sheriff Michael Slupe.

Slupe, who was inside the rally venue when the shooting erupted and did not witness the encounter, said the officer could not have wielded his own gun under the circumstances.

“I think all law enforcement on site did everything that they could, especially the local law enforcement,” Slupe said on Monday. “I hope they’re not made a scapegoat, because they did their job to the best of their abilities.”

The FBI said it was investigating the attack as a potential act of domestic terrorism, but the agency had not identified a clear ideological motive. The FBI believes Crooks, who had bomb-making materials in the car he drove to the rally, acted alone.