Three U.S. Secret Service officials were placed on administrative leave Tuesday, pending the outcome of an investigation by the agency’s professional standards office.

The three were operating in non-law-enforcement capacities and were being investigated for potential misconduct, the agency’s public information office reported. However, the cause for the action remained unclear as the Office of Professional Responsibility was beginning its review.

The Secret Service has been under scrutiny as President Donald Trump has been the subject of at least three assassination plots in the past two years. In April, police said an armed man breached security at the Washington Hilton during the White House Correspondents' Association dinner, which Trump was attending.