At least five U.S. Secret Service officials involved in the planning of Donald Trump's rally in Butler, Pennsylvania, on July 13, where a gunman attempted to kill the former president, have been placed on administrative duty, according to two sources familiar with the situation.

The officials are still working but not allowed to be involved in anything operational including the security planning, the sources told ABC News.

The Secret Service's Office of Professional Responsibility investigation has been intensifying, and while the investigation has focused on the failures, miscues and planning up to Butler, the probe is now focusing on a number of issues, including the number of classified threats against former President Trump by Iran and what the agency did in response security-wise regarding the new intelligence.

An official tells ABC News this means even more senior officials might be caught up in the review.

"The U.S. Secret Service is committed to investigating the decisions and actions of personnel related to the event in Butler, Pennsylvania and the attempted assassination of former President Donald Trump," the agency said. "The U.S. Secret Service's mission assurance review is progressing, and we are examining the processes, procedures and factors that led to this operational failure."

The agency said it holds its personnel to the "highest professional standards" and would not comment further.

Trump was struck in his ear by a bullet during the assassination attempt at his campaign rally on July 13, which also killed one spectator and injured two others, according to officials. The gunman, identified by the FBI as 20-year-old Thomas Matthew Crooks, was killed by Secret Service agents.

The U.S. Secret Service recently made arrangements for Trump to resume outdoor campaign rallies by surrounding his podium with bulletproof glass, multiple sources told ABC News.

Editor's note: This story has been corrected to note that the Secret Service officials were put on administrative duty, not leave.