The United States Secret Service is a federal agency dedicated to overseeing the protection and safety of former/current national leaders and their families. Secret Service personnel are also trained in preventing and investigation forged or counterfeit US dollar notes.
In addition, personnel working with or alongside the President must have "Yankee White" security clearance to do so.
History on NCISEdit
The Secret Service appeared in the debut NCIS episode, Yankee White (episode) where Special Agent Caitlin Todd was revealed to be a Secret Service assigned to the Presidential Protection Detail on Air Force One with her boss, William Baer in charge.
After the investigation was closed, Todd resigned from the Secret Service to accept a job at NCIS where she stayed for a year and a half prior to her murder at the hands of rogue sniper Ari Haswari in the Season 2 finale, Twilight (episode)
The Secret Service has not been seen or heard since Yankee White but Tobias Fornell mentioned them during a conversation with Leroy Jethro Gibbs in Reveille (episode) although another Secret Service agent later appeared in the Season 10 episode, You Better Watch Out (episode).
Mark Harmon (Jethro Gibbs) and Michael O'Neill both previously played Secret Service agents in the American TV series, the West Wing.