Washington DC - Lawmakers probing the Pennsylvania assassination attempt against Donald Trump blamed Secret Service failures during their first hearing Thursday for a series of security lapses that allowed a gunman to open fire on the Republican presidential nominee.

The Secret Service initially blamed local law enforcement over the July 13 attack at Trump's campaign event in Butler, which left one man dead and the ex-president nursing a bloodied ear where a bullet grazed him.

But members of a cross-party House task force set up to investigate the incident heard that the agency, which protects US political leaders, communicated poorly over securing the nearby building that gunman Thomas Michael Crooks scaled and took aim from.

Witnesses from several law enforcement agencies described chaotic and piecemeal organization in the hours before Trump took the stage, with no central command post in charge of messaging and instructions shared haphazardly.

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"In the days leading up to the rally, it was not a single mistake that allowed Crooks to outmaneuver one of our country's most elite group of security professionals – there were security failures on multiple fronts," said Republican co-chair Mike Kelly, whose hometown is Butler.

His Democratic counterpart Jason Crow praised local police but said communication between all the agencies involved had been "disjointed and unclear."

The panel was set up days after the shooting and has carried out two dozen interviews with local police, met with federal agents, and received more than 2,800 pages of documents from the Secret Service. 

Its seven Republicans and six Democrats visited the rally site in August, meeting local law enforcement officials.

The hearing came in the wake of a scathing report by a Senate committee focusing on a lack of clear decision-making and leadership over who was responsible for each section of the rally site.

Texas Republican Pat Fallon told Thursday's hearing that a "10-year old" looking at a satellite image could have seen that the roof the gunman used as the base for his attack was the venue's greatest security threat.

Drew Blasko, an assistant leader of the Butler County Emergency Services Unit's sniper team, said he had spoken to Secret Service agents before the rally to share his concerns about the building and asked the agency to post additional people there.

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The panel recently widened its remit to take in an apparent second assassination attempt on Trump earlier in September, after another gunman appeared to be setting up to shoot the Republican while he was on his golf course in southern Florida.

It is expected to recommend legislative reforms in a December report.

Secret Service Director Kimberly Cheatle resigned after appearing at an earlier congressional hearing on the Pennsylvania attack and acknowledged the Secret Service's "most significant operational failure" in decades.

A government funding bill approved by Congress this week includes an extra $231 million for the Secret Service, although some lawmakers have demanded a wide-ranging overhaul of the agency.