Washington DC - Under growing pressure for a massive security failure, the Secret Service on Monday 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 Saturday, a brazen attack that shocked a nation already deeply polarized ahead of the November vote.

"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 Biden yesterday and will participate fully. We will also work with the appropriate Congressional committees on any oversight action," Cheatle added.

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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 were injured.

Biden ordered a full review of 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.

The Secret Service has been under growing pressure over how a gunman aiming an assault rifle was allowed to take position on a roof some 500 feet from one of the most protected political figures on the planet.

And the questions grew louder when phone footage emerged of people sighting the gunman on the roof and trying to warn security before he opened fire.

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

The Secret Service "designs operational security plans for National Special Security Events (NSSE) to be dynamic in order to respond to a kinetic security environment and the most up-to-date intelligence from our partners," her statement said.

On Sunday the Secret Service said it was "fully prepared" for the convention.