The U.S. Secret Service's new acting director said in prepared congressional testimony on Tuesday he was "ashamed" of a major security lapse that preceded the July 13 attempted assassination of Republican presidential nominee Donald Trump at a Pennsylvania campaign rally.

In testimony released before addressing two Senate committees, Acting Secret Service Director Ronald Rowe said he visited the outdoor rally site in Butler and climbed onto the roof of a nearby building from which 20-year-old Thomas Crooks fired shots that wounded Trump's right ear, killed one rally attendee and wounded two others, with an AR-15-style rifle.

"What I saw made me ashamed," Rowe said in the prepared testimony for 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."

Rowe's admission of a glaring security lapse came a week after former Secret Service Director Kimberly Cheatle resigned under bipartisan congressional pressure after refusing to detail security shortfalls in testimony before a House of Representatives panel.

Rowe sought to assure lawmakers that he has since taken steps to prevent similar lapses from occurring amid concerns among both Democrats and Republicans about further political violence as the campaign intensifies ahead of the Nov. 5 U.S. election.

"I directed our personnel to ensure every event site security plan is thoroughly vetted by multiple experienced supervisors before it is implemented," Rowe said.

The Secret Service, a federal law enforcement agency whose duties including protecting the president and certain other top political figures, has added six people to its protection list since July 13, including Republican vice presidential candidate JD Vance and his family and independent presidential candidate Robert F. Kennedy, while reinforcing security details, Rowe said.

Rowe was due to testify alongside Paul Abbate, deputy director of the FBI, which is due to interview Trump on Thursday as part of its investigation into the shooting.

The attempted assassination is the topic of multiple investigations by House and Senate committees, as well as a new bipartisan task force established by Republican House Speaker Mike Johnson and House Democratic leader Hakeem Jeffries.

Investigators have been unable to determine a clear motive for Crooks, who they have described as a loner with no close friends and a social network limited mainly to immediate family members. Much interest revolves around the timeline from when law enforcement officials first noticed Crooks to when Secret Service snipers killed him after he opened fire.

The FBI said on Monday that Crooks first came to the attention of police more than an hour before he fired at least eight shots at the former president.

A local officer took a photo of Crooks and sent it to other law enforcement officials at the scene. About 30 minutes later, SWAT team members saw Crooks using a rangefinder and browsing news sites, the FBI said.