The U.S. Secret Service is facing harsh public scrutiny and investigations as the assassination attempt of former U.S. president Donald Trump thrusts the agency into the centre of a political firestorm.

The Secret Service is a unit of the Department of Homeland Security and provides round-the-clock security to the president and former presidents.

Speaking on CNN on Monday, DHS chief Alejandro Mayorkas said he had full confidence in the leadership of the Secret Service, but said the shooting amounted to a failure.