WASHINGTON, July 16 — Facing growing criticism over a massive security failure, the US Secret Service yesterday vowed to cooperate with an independent review after a shooter was allowed to open fire on Donald Trump.

The 78-year-old former president was injured but survived an assassination attempt at a campaign rally in Pennsylvania on Saturday, a brazen attack that shocked a nation already deeply polarised ahead of the November election.

"The Secret Service is working with all involved federal, state and local agencies to understand what happened, how it happened, and how we can prevent an incident like this from ever taking place again,” the agency’s director Kimberly Cheatle said in a statement.

"We understand the importance of the independent review announced by President (Joe) Biden yesterday and will participate fully,” Cheatle added.

Trump was speaking at a rally in Butler, Pennsylvania, when multiple bangs rang out.

He clutched his ear, with blood visible on his ear and cheek, then ducked to the floor as Secret Service agents swarmed onto the podium, surrounding him and rushing him to a nearby vehicle.

The shooter and a bystander were killed, and two spectators injured.

Newly surfaced video backs up reports from witnesses that they had called out to police and physically pointed at the shooter as he lay on the roof preparing to open fire, the Washington Post reported Monday.

The shots targeting Trump rang out 86 seconds after the first audible attempts to warn the police, the Post said, citing an analysis it did of video clips from the scene of the attack.

Biden ordered a full review of the security at the rally, as well as at this week’s Republican National Convention in Milwaukee, Wisconsin, where Trump will be crowned the party’s presidential nominee.

He also ordered Secret Service protection for independent candidate Robert F. Kennedy Jr., an environmental lawyer and long-time vaccine skeptic who has no chance of winning in November, but whose candidacy could potentially sway close contests in key swing states.

Heightened risk

The Secret Service faces intense scrutiny over how a gunman aiming an assault rifle was able to take position on a roof some 500 feet (150 metres) from one of the most protected political figures on the planet.

With Trump set to star at the convention, Cheatle said the agency was working to toughen security.

The Secret Service designs plans for major events "to respond to a kinetic security environment and the most up-to-date intelligence,” her statement said.

Earlier in the day, Trump had called for Secret Service protection for Kennedy "in light of what is going on in the world today.”

"Given the history of the Kennedy Family, this is the obvious right thing to do!” Trump wrote on his Truth Social web site.

Kenedy’s uncle, President John F. Kennedy, was assassinated in Dallas, Texas in 1963. Five years later, the candidate’s father, Robert, was shot to death in Los Angeles while on the campaign trail.

The US Secret Service is responsible for the safety of the president, vice president and former presidents, and their families, as well as major election candidates and visiting foreign heads of state. — AFP