The strains are occurring as the agency shifts focus to safeguarding the Republican National Convention this week in Milwaukee.

In her interview with ABC News, Cheatle appeared to shift some of the blame to local authorities, who coordinated with the Secret Service on Saturday. She said agents moved quickly to stop the shooter after rallygoers spotted him atop a building just outside the security perimeter, but that police officers were inside that structure while the gunman, identified as 20-year-old Thomas Matthew Crooks, was on the roof.

The Secret Service was tasked with securing the inner perimeter of the event, while the local police were assigned to safeguard the area that included the building, Cheatle noted.

The Secret Service is responsible for co-ordinating the overall security plan for events where the president, former president and other senior officials it protects attend. It also has primary responsibility for addressing the risk of a long-range shooter and blocking lines of sight, including by instructing local police to secure or stand guard at buildings that could provide an opportunity for an attacker.

“There was local police in that building – there was local police in the area that were responsible for the outer perimeter of the building,” she told ABC News.

Butler Township police referred inquiries to Pennsylvania State Police, which said in a statement that it “provided all resources” the Secret Service requested. The state police referred questions about the investigation to the FBI.

Mayorkas said that in the aftermath of the weekend’s violence in western Pennsylvania, the Secret Service adjusted its protective measures for Trump, President Biden, Vice President Harris and others, and that security details would be provided to independent candidate Robert F Kennedy Jr along with Trump’s newly announced running mate, Senator J.D. Vance (R-Ohio).

Additionally, officials will assemble a team to conduct an independent review of the apparent lapses on Saturday, Mayorkas said.

“We need to move with swiftness and urgency because this is a security imperative,” he told reporters, adding that the work should commence within “days.”

Lawmakers are launching inquiries of their own. Rep James Comer (R-Kentucky), chairman of the House Oversight Committee, led a group of Republicans who wrote to Cheatle asking by Thursday for a list of the agents and officers assigned to safeguard the rally, as well as any recording of what transpired and directives from the agency’s leadership in response to the shooting.

The letter asks for Cheatle to furnish additional materials by July 29 that include security assessments, reports and plans related to the agency’s preparation for the event.

“Americans deserve answers,” Comer wrote on social media, saying he expects Cheatle to testify before the committee on July 22.

The House Oversight Committee will receive a briefing on the assassination attempt on Tuesday, according to a person familiar with the planning.

Mayorkas said the independent review would be led by someone outside of the Department of Homeland Security and “externally of the government, so that no question of its independence can be raised.” Their findings will be made public, he said.

“It’s very important,” he added, “that this independent review have the confidence of the people.”

Mayorkas said it would examine the actions of Secret Service and other law enforcement personnel “before, during and after the shooting.”

“An incident like this cannot happen,” Mayorkas told CNN earlier in the day. “We are speaking of a failure.”

Cheatle has said the agency would cooperate fully with an independent review and “work with the appropriate congressional committees on any oversight action.”

The US Secret Service is part of the Department of Homeland Security. Biden appointed Cheatle as director in 2022 and noted that she had served on his security detail when he was vice president.

The agency has more recently come under scrutiny for failing to preserve its text messages for an investigation into the January 6, 2021, assault on the US Capitol by Trump supporters. A 2015 House Oversight Committee report found that the Secret Service had stumbled during multiple security breaches, including a 2011 shooting at the White House under the Obama administration.

Senators Gary Peters (D-Michigan) and Rand Paul (R-Kentucky), the chairman and ranking minority-party member of the Senate Homeland Security and Governmental Affairs Committee, said on Monday that the panel will conduct a bipartisan investigation of the shooting. The senators plan to hold a hearing “to examine security failures” that led to the shooting, the committee announced.

House Homeland Security Committee Chairman Mark Green (R-Tennessee) has also asked Cheatle for a list of documents.

The Secret Service agents who protected Trump and killed his attacker have been widely praised for acting quickly and decisively. But it remains unclear how Crooks was able to gain access to a nearby roof with an AR-style rifle.

Leon Panetta, a former CIA director and defence secretary during the Obama administration, said the Biden administration should urgently assess the failures at the Trump rally to prevent another attack in the coming months, as the nation remains deeply divided in the lead-up to November’s election.

“Somebody’s got to head up this investigation as quickly as possible,” Panetta, now a member of the Homeland Security Advisory Council, said in a telephone interview. “We’re not at the end of a political campaign, we’re at the beginning of a political campaign. We’ve got to quickly learn what went wrong and make sure that we’ve corrected it.”

When he was White House chief of staff in the Clinton administration, Panetta said, he would meet with the Secret Service before presidential events to ensure they had considered every possible scenario and prepared for contingencies.

Bill Bratton, a former New York police commissioner and a member of the Homeland Security Advisory Council, said he expected the FBI-led criminal investigation to take priority over the administrative probe into the Secret Service’s actions. Authorities must conclusively and urgently determine if the shooter acted alone – which Bratton said appears increasingly likely – or in concert with any terrorist organizations, he said.

Bratton praised the Secret Service agents who protected Trump during the attack. But he said the agency’s preparations before the rally appear “deficient” and “not the Secret Service’s finest hour.”

“Nobody is questioning the bravery and the actions after the event began to unfold, the Secret Service putting their bodies between the sniper and the president,” he said. “It’s the run-up that’s really going to be the focus. There’s a potential for people to lose their positions as a result of this, certainly.”

White House press secretary Karine Jean-Pierre told reporters that Cheatle has Biden’s support and confidence. Mayorkas said he did not believe that anyone had been reassigned or removed from Trump’s security detail.

Emilio T Gonzalez, a former Army intelligence officer and Homeland Security official, said the shooting revealed flagrant flaws in the Secret Service’s planning and resources. He said he thought much of Trump’s security detail should be replaced since the shooting.

“This was terribly, terribly organised from a security perspective,” Gonzalez, who supports Trump, said in an interview.

“This guy should never have been that close to President Trump. His Secret Service protection should have been airtight, and it wasn’t,” he added. “Then the question is: Why wasn’t it?”