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.Video above: Former Secret Service agent speaks after Trump rally attackWhen 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.Stan Kephart, a former police chief who worked event security for two former presidents, said the shooting followed an "an absolute and abysmal failure" on the part of the Secret Service to protect Trump. The agency is ultimately responsible for the candidate's safety, he added."You don't get to blame other people. They are under your control," said Kephart, now a consulting expert on law enforcement event security.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."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 told The Associated Press on Monday. "I hope they're not made a scapegoat, because they did their job to the best of their abilities."Butler Township Manager Tom Knights said the officer lost his grip and was not retreating when he fell 8 feet (2.4 meters) to the ground."He was literally dangling from the edge of a building and took the defensive position he needed to at that time. He couldn't hold himself up," Knights said.The officer, who has 10 years of experience in law enforcement, severely injured an ankle in the fall and was in a walking boot, Knights said.Two spectators were critically wounded in the shooting. A former fire chief, 50-year-old Corey Comperatore, was killed.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.The FBI said Monday that analysts were able to gain access to the shooter's phone, though a law enforcement official who was not authorized to discuss the matter publicly and spoke on condition of anonymity said the device has not revealed meaningful information about any potential motive.The official also said that investigators believe Crooks purchased 50 rounds of ammunition on the day of the attack.The FBI has conducted nearly 100 interviews of law enforcement officials, attendees at the rally and other witnesses, and it has received hundreds of digital media tips.Authorities said they believe the gunman's AR-style rifle was purchased by his father. Kevin Rojek, FBI special agent in charge in Pittsburgh, said investigators do not yet know if Crooks took the gun without his father's permission.The roof where Crooks lay was an estimated 147 yards (135 meters) from where Trump was speaking, a distance from which a decent marksman could reasonably hit a human-sized target. That is a distance at which U.S. Army recruits must hit a scaled human-sized silhouette to qualify with the M-16 rifle.Patrick Brosnan, a former New York Police Department detective who runs a national private intelligence and security firm that has protected heads of state, suggested a building that close to the stage should have been secured and under surveillance from the start. An AR-15-type weapon has an effective range of about 400 to 500 yards (366 to 457 meters). The gunman was under 150 yards (137 meters) from Trump."It would just seem to me to be elemental and rudimentary as it relates to providing a steel band or circular band of protection for the former president, who is fully exposed," said Brosnan, who has trained with the Secret Service and spoken in the past to Trump and his family about security issues.___Associated Press writers Jesse Bedayn in Denver and Michael Sisak in New York contributed to this report.