A new video with a graphical reticule shows US Republican presidential candidate Donald Trump's head in the middle of what would have been the sniper's crosshair.

The video shows Mr Trump's tilting his head to look at a screen graphic and leaning into the microphone saved his life, according to a post on X by US journalist Mario Nawfal, whose 'The Roundtable Show' is billed as the biggest example of citizen journalism.

"New footage shows Trump dodging a perfectly aimed shot. Close-up footage reveals Thomas Crook's shot was perfectly centred on Trump's head... The footage was captured just seconds before the shot," Mr Nawfal said in the post.

The US Department of Homeland Security is investigating the Secret Service's handling of security at the campaign rally where the gunman tried to assassinate Mr Trump.

The Secret Service has faced intense scrutiny since Saturday's shooting in Butler, Pennsylvania, during which a gunman opened fire on the Republican presidential candidate from an exposed rooftop some 150 metres away from the stage where Mr Trump was speaking.

Mr Trump was wounded in the ear and a rally attendee was killed. The 20-year-old gunman, Thomas Matthew Crooks, was shot dead by Secret Service snipers.

President Joe Biden has ordered an independent review of the Secret Service's handling of the assassination attempt, which is also being investigated by the Federal Bureau of Investigation (FBI) as a case of potential domestic terrorism.

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