Washington DC - The Secret Service on Friday detailed a litany of security failures uncovered by its review of the attempted assassination of former president Donald Trump at a rally in July.

Shooter Thomas Matthew Crooks was able to open fire from a nearby rooftop at the outdoor event held by Republican election candidate Trump, who narrowly escaped death and suffered a wound to his right ear.

The review "identified deficiencies in the advanced planning and its implementation by Secret Service personnel," Acting Director Ronald Rowe said at a press briefing.

"While some members of the advance team were very diligent, there was complacency on the part of others that led to a breach of security protocols."

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Among the failures identified by Rowe were poor communication with local law enforcement, an "over-reliance" on mobile devices "resulting in information being siloed," and line of sight issues, which "were acknowledged but not properly mitigated."

"At approximately 18:10 local time, by a phone call, the Secret Service security room calls the countersniper response agent reporting an individual on the roof of the AGR building," Rowe said.

"That vital piece of information was not relayed over the Secret Service radio network."

Two attendees of the rally in Butler, Pennsylvania, were injured from gunfire, and a third, 50-year-old firefighter Corey Comperatore, died as a result.

Crooks was shot dead by Secret Service personnel.

Secret Service director Kimberly Cheatle resigned in the aftermath of the incident and several additional Secret Service agents have been put on leave.