An independent panel reviewing the failures that led to the attempted assassination of former President Donald J. Trump in July called on the Secret Service to replace its leadership with people from the private sector and focus almost exclusively on its protective mission.

The recommendations, part of a report released Thursday commissioned by the Department of Homeland Security, outlined deficiencies that have already been identified in the months after the rally in Butler, Pa., on July 13. Those include the failure of the Secret Service to secure a nearby building where a rooftop would-be assassin fired eight shots toward Mr. Trump. That and other security lapses, members of the panel said, result from an absence of “critical thinking” among agents and supervisors.

The panel was particularly struck by a “lack of ownership” conveyed by the agents it interviewed. Those involved in the security planning did not take responsibility in the lead-up to the event, nor did they own failures in the aftermath. And, the report added, they “have done little in the way of self-reflection in terms of identifying areas of missteps, omissions or opportunities for improvement.”

The findings are stark but familiar. A panel convened in 2014 after a man scaled the White House fence and entered the mansion made similar proposals. That the issues persist a decade later underscores the challenge of overhauling an agency with such an entrenched culture.

“The service has become insular and stale,” Janet Napolitano, a member of the four-person panel, said in an interview. Ms. Napolitano was the secretary of the Department of Homeland Security, the Secret Service’s parent agency, from 2009 to 2013. “It is time for the service to kind of break out and to reach out beyond its own agency to bring in talent that can really take a fresh look at what it is they do, and how they do it.” In the past century, the agency has had only one director not promoted from within.

The panel included Frances Fragos Townsend, a former homeland security adviser during the George W. Bush administration; David Mitchell, who has been in law enforcement for more than 50 years; and Mark Filip, a former deputy attorney general during the Bush administration. They conducted 58 interviews and reviewed more than 7,000 documents.

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