The U.S. Secret Service is facing harsh public scrutiny and investigations as the assassination attempt of former President Donald Trump thrusts an agency with a checkered past into the center of a political firestorm.

Since Saturday’s shooting at a campaign rally in Butler, Pennsylvania, Trump allies have blasted the agency over its protection of the former president. Security concerns are particularly acute as thousands descend on Milwaukee for the start of the Republican National Convention on Monday.

Although the Secret Service, a unit of the U.S. Department of Homeland Security, provides round-the-clock security to former presidents, Trump’s appearances at mega rallies have posed unique challenges. Some critics have claimed the agency rejected requests from the former president’s team for more protection — a charge that the agency called “absolutely false.”