"There was complacency on the part of" some agents "that led to a breach of security protocols", Acting Secret Service Director Ronald Rowe told reporters on Friday, adding agency employees would be held accountable.
"These penalties will be administered according to our disciplinary process," Rowe said, declining to provide specifics.
The assassination attempt against Trump at the rally in Butler on July 13 led to widespread criticism of the Secret Service and the resignation of its former director.
Critics raised concerns about how the suspect was able to access a nearby rooftop with a direct line of sight to where the former president was speaking.
Rowe defended the Secret Service's actions in a separate apparent assassination attempt against Trump at his Florida golf club on Sunday.
In that case, a Secret Service agent fired at a suspect after spotting a rifle poking through the tree line.
The suspect did not fire a shot and did not have a line of sight to Trump, who was golfing a few hundred metres away.
The suspect, identified as Ryan Routh, 58, fled.
He was later arrested and charged with gun offences, and is expected to appear in federal court on Monday.
"It appears that those agents, those supervisors, made swift decisions and made correct decisions," he said of the Sunday attempt.
Trump's ear was wounded, a rally attendee was killed and two others were wounded in the July shooting.
The Secret Service has since said it was "ashamed" of the security lapse.
Rowe said Trump was getting the same level of protection as President Joe Biden and Democratic presidential candidate Vice-President Kamala Harris, his rival in the November 5 election.
The Secret Service's internal investigation identified communication lapses between state and local enforcement.
The site in Butler was identified as a security challenge by the Secret Service, but measures designed to alleviate those concerns were not "carried out as intended", according to a summary of the findings.
Trump's security detail was not aware that state and local law enforcement were intensely pursuing a suspicious person, who turned out to be the gunman.
Had they been aware, agents might have moved Trump to another location during the search, according to the report.
The Secret Service probe is in addition to other probes led by Congress, an independent panel convened by the Department of Homeland Security and the department's internal watchdog.
