WASHINGTON — The Secret Service’s acting director on Tuesday told lawmakers he considered it indefensible that the roof used by the gunman in the attempted assassination of former President Donald Trump was unsecured and said it was regrettable that local law enforcement had not communicated to his agency that a gunman had been spotted on a nearby roof.

Ronald Rowe also testified that he recently visited the shooting site and laid down on the roof of the building where shots were fired in order to evaluate the gunman’s line of sight during the July 13 shooting in Butler, Pennsylvania.

“What I saw made me ashamed. As a career law enforcement officer and a 25-year Secret Service veteran, I cannot defend why that roof was not better secured,” he said.

The testimony was the most detailed catalog to date by the Secret Service of law enforcement failings and miscommunications, with Rowe accepting blame for his own agency’s mistakes while also pointedly criticizing local law enforcement for communication breakdowns that resulted in his agency not sharing information that a gunman, later identified as 20-year-old Thomas Matthew Crooks, had been spotted on the roof of a building less than 150 yards from the rally stage where Trump was speaking.

“Neither the Secret Service counter sniper teams nor members of the former president’s security detail had any knowledge that there was a man on the roof of the building with a firearm,” Rowe said. “It is my understanding those personnel were not aware the assailant had a firearm until they heard gunshots.”

He said that the shooting amounted to a “failure on multiple levels,” including a failure of imagination and a “failure to challenge our assumptions.”

“We assumed that the state and locals had it,” Rowe said. “We made an assumption that there was going to be uniformed presence out there, that there would be sufficient eyes to cover that, that there was going to be counter-sniper teams” in the building from whose roof Crooks fired shots.

“And I can assure you,” Rowe added, “that we’re not going to make that mistake again.”

He said he had implemented multiple reforms since taking over as acting director last week, including mandating that every event security plan is vetted by multiple experienced supervisors before being implemented, expanding the use of aerial drones to improve visibility of roofs and dedicating more resources to improve communications at events where the Secret Service is operating.

Trump, the 2024 Republican presidential nominee, was struck in the ear by a bullet or a bullet fragment in the assassination attempt, one rallygoer was killed and two others were injured before the gunman was killed by a Secret Service counter-sniper.

The blunt and at times emotional testimony Tuesday, featuring combative exchanges with lawmakers, ensured that an already simmering blame game between federal and local authorities will continue. It also suggested that Rowe, with ready and generally detailed answers, was determined to strike a different posture than that of his predecessor, Kimberly Cheatle, who resigned last week after facing intense criticism from lawmakers from both major political parties following responses at a congressional hearing that were seen as evasive and lacking in specifics.

Tuesday’s hearing before the Senate Judiciary and Homeland Security committees was the latest in a series of congressional sessions dedicated to the law enforcement lapses and missed communications that preceded the shooting.

Local law enforcement officers had first observed a suspicious-looking man at the rally site more than an hour before the event and circulated that information, including photographs of a man who turned out to be Crooks. But the officers ultimately lost track of Crooks, who was able to scale the roof of a building at AGR International Inc., a supplier of automation equipment for the glass and plastic packaging industry, and fire an estimated eight shots with an AR-15-style rife.

Shortly before the shooting, a local officer climbed up to the roof to investigate. Crooks turned and pointed his rifle at the officer, who retreated.

Even though text messages among local snipers revealed anxiety about the man, Rowe said the only thing the Secret Service knew at the time of the shooting was that law enforcement was contending with a suspicious-looking man.