The assassination attempt against former US president Donald Trump at a Pennsylvania rally in July was “preventable and should not have happened”, according to a bipartisan investigation into the shooting and the “stunning security failures” surrounding the event.

The report from a House task force, released on Monday, is just the latest look at the wide-ranging law enforcement failings that preceded the July 13 shooting at a rally in Butler, Pennsylvania, where Mr Trump was struck in the ear by gunfire.

One rallygoer was killed in the shooting and two others were wounded.

US election opinion polls
(PA Graphics)

Members of both the US House and Senate have repeatedly questioned why the Secret Service, an agency tasked with protecting the country’s top leaders, did not do a better job communicating with local authorities during the campaign rally, particularly when it came to securing the building that was widely agreed to be a security threat but that ultimately was left so unprotected that gunman Thomas Matthew Crooks was able to access it and open fire.

Legislators in their report focus on “the fragmented lines of communication and unclear chains of command” between the Secret Service and Pennsylvania state and local police, but place the majority of the blame on the Secret Service for the security breakdown.

“Federal, state, and local law enforcement officers could have engaged Thomas Matthew Crooks at several pivotal moments,” the report stated.

Donald Trump at a baseball game
Mr Trump is looking to return to the White House (AP)

Legislators added that throughout the afternoon, “as Crooks’s behaviour became increasingly suspicious, fragmented lines of communication allowed Crooks to evade law enforcement” and climb onto the unsecured roof where he would eventually start shooting.

“Put simply, the evidence obtained by the Task Force to date shows the tragic and shocking events of July 13 were preventable and should not have happened,” the report continued.

The preliminary findings are drawn from thousands of pages of documents, nearly two dozen transcribed interviews with state and local officials as well as a series of both classified and non-classified briefings from senior officials at the Secret Service and the FBI.

The report breaks little new ground, as the failed Secret Service response has been already documented by an independent commission, an interim Senate report as well as congressional testimony and news media investigations. The House report, like others before it, does not identify specific individuals who may be to blame.

But The Associated Press has previously reported that at least five Secret Service agents have been placed on modified duty.

The director of the Secret Service at the time, Kimberly Cheatle, resigned shortly after the shooting, saying she took full responsibility for the lapse.

The task force – comprising seven Republicans and six Democrats – showcased some of the report’s findings during a public hearing last month. Lawmakers say they plan to issue a final report, including recommendations to avoid future assassination attempts against political candidates, by mid-December.

The task force has also begun investigating the second assassination attempt on Mr Trump last month where a man with a rifle camped outside one of his golf courses in southern Florida.