Multiple Secret Service agents involved in planning security for Donald Trump’s July 13 campaign rally in Butler, Pennsylvania, failed to properly mitigate line-of-sight risks to the stage that led to the assassination attempt on the former president, the agency’s internal inquiry found.

The Secret Service has completed its Mission Assurance Inquiry of the assassination attempt on Mr. Trump, the Republican presidential nominee, and identified numerous failures and “deficiencies” in planning, communication and command and control processes.

The agency shared a classified report documenting the inquiry’s findings with various oversight entities, including congressional investigators. The Secret Service’s Office of Professional Responsibility, Inspection Division conducted the inquiry.

A seven-page unclassified summary of the report is all that was released publicly. It provides a few new tidbits of information while largely rehashing issues from other probes.

One new element is the Secret Service taking accountability for failing to mitigate a wide open view to the rally stage from the adjacent AGR International complex where the would-be assassin, Thomas Crooks, climbed onto the roof of a group of interconnected warehouses and fired at Mr. Trump as he spoke.

“Multiple Secret Service personnel mistakenly assessed these line-of-sight risks to the former president as acceptable, leading to inadequate elimination,” the report said.

Agency supervisors reported that they expected large pieces of farm equipment would be placed between the AGR complex and the rally site to block the line of sight to the stage, but they ultimately were not used.

“Site advance personnel failed to recognize gaps in the site’s construction and failed to notify their chain of command that potential line-of-sight concerns were not fully mitigated,” the report said.

Another new admission in the Secret Service report is that the agency “did not command the appropriate dispatch of personnel, either Secret Service or state/local law enforcement partners, to the area of the AGR building after learning about a suspicious individual with a range finder.”

At least a half hour before the rally began, local countersnipers assisting with security spotted Crooks on the AGR complex using a rangefinder to view the distance to the stage.

That information was ultimately passed via telephone and text messages, so it reached only select Secret Service agents since it was never shared widely over radio communication channels.

Communication and coordination issues between the Secret Service and its local law enforcement partners securing the Butler rally have been well documented through various probes into the assassination attempt.

One of the biggest coordination failures was the siloing of command posts, with local law enforcement from the Butler County Emergency Service Unit in a separate post from the Secret Service security room.

The Secret Service said it “planned for” multiple public safety agencies supporting the event to be colocated in its security room to monitor radio transmissions and share information, but agency personnel failed to ensure that staffing plan was executed according to protocol.

“Two state and local agencies supporting the Butler rally with prior experience working with the Secret Service observed that agency planning for the Butler rally lacked detail, cohesion and clear understanding of who was in charge, all of which contributed to the overall lack of coordination,” the report said.

The Secret Service also used Homeland Security Investigations special agents to help staff the rally, noting that some reported “they were inadequately prepared due to insufficient instruction.”

The report added that “a higher-than-expected proportion” of security personnel were diverted to assist with medical responses, many resulting from the extreme heat at the outdoor rally.

While Secret Service personnel identified a need for additional water, coolers, volunteers and improved access for public safety personnel to administer medical care, the report said there was a “lack of coordination with campaign staff on these issues.”

The internal review “identified several instances of behaviors and acts by multiple employees that warrant review for corrective counseling and, potentially, disciplinary action,” the report said.

The Secret Service’s Office of Integrity will review the inquiry findings, and “all individuals found in violation of policies will be held accountable.”

A news release from the Secret Service said Ronald Rowe, its acting director, “has developed a plan for an agencywide paradigm shift that will address the issues identified in the report and help the agency meet the increasing demands of a dynamic threat environment.”

That includes personnel retention and recruitment, technology modernization and training improvements that will require additional funding.

“The Secret Service is working with Congress to dramatically increase the agency’s resources so that we can make this paradigm shift a reality and ensure that a tragedy like July 13 never happens again,” the news release said.

The summary report listed some of the changes the Secret Service has made since the assassination attempt, including increasing special agents assigned to Mr. Trump’s protective detail and expanding technical countermeasures used to provide security for him and his residence.

The agency has also made changes addressing communication issues, including improved protocols to ensure proper colocation of law enforcement partners in event command posts and radio interoperability.

The bipartisan House task force investigating the assassination attempt released a statement saying the Secret Service’s internal findings align with information uncovered in the task force’s ongoing investigation.

“Many of the steps to improve security outlined in the review appear to be headed in the right direction,” the task force said. “However, the Secret Service still needs to take additional steps to create accountability and ensure that the mistakes that occurred on July 13 are not repeated.”