At least five Secret Service agents have been placed on modified duty after the failed attempt to assassinate US presidential candidate Donald Trump. 

They include the special agent in charge of the Pittsburgh field office, three other agents assigned to the office, and one agent who was assigned to Trump's protective detail, a law enforcement official said. 

The Pittsburgh office was responsible for the security planning ahead of the Trump rally in Butler, Pennsylvania, according to an official who had direct knowledge of the matter. 

The agents are on administrative leave, meaning they cannot perform investigative or protective work.

On July 13, 20-year-old Thomas Crooks used an AR-15-style rifle to fire shots that hit Trump's right ear, kill one rally attendee and injure two others.

The shooting was a devastating failure to carry out one of the agency's core duties and it led to the resignation of the Secret Service's then-director, Kim Cheatle.

At a congressional hearing after the assassination attempt, Ms Cheatle acknowledged the Secret Service was told about a suspicious person two to five times before the shooting. 

She also revealed the roof from which Mr Crooks opened fire was identified as a potential vulnerability days before the rally.

The US Secret Service's new acting director, Ronald Rowe Jr, said he was "ashamed" of the security lapse that led to the attempted assassination but blamed the shortfall on local law enforcement.

In testimony before two Senate committees in July, Acting Director Ronald Rowe said he visited the outdoor rally site in Butler and climbed onto the roof of the building from which Mr Crooks fired shots. 

"What I saw made me ashamed," Mr Rowe told a joint hearing of the Senate Homeland Security and Judiciary Committees. 

"As a career law enforcement officer and a 25-year veteran with the Secret Service, I cannot defend why that roof was not better secured." 

AP