Washington DC - The Department of Homeland Security's inspector general said Wednesday that it is investigating the Secret Service's handling of security at the campaign rally where a gunman tried to assassinate former president Donald Trump.

The office of DHS Inspector General Joseph Cuffari said in an online posting that the probe is intended to "evaluate the United States Secret Service's process for securing" the campaign event.

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 yards away from the stage where Trump was speaking.

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.

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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 FBI as a case of potential domestic terrorism.

Members of the House of Representatives and Senate are to receive briefings from the Secret Service, the Justice Department, and the FBI later Wednesday on the status of the various probes.

The House Judiciary Committee on Wednesday announced plans to hold a hearing on July 24 into the FBI's investigation into the assassination attempt, and Republican House Speaker Mike Johnson told Fox News he would seek the resignation of Secret Service director Kimberly Cheatle.

Cheatle is scheduled to appear before the House Committee on Oversight and Accountability on Monday for a hearing into the assassination attempt.

The 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.