The Sentinel (2006) on Netflix

Pete Garrison is a Secret Service Agent. Another agent wanted to talk to him but couldn't at the time. Later the man is killed. One of Pete's informants tells him that there's a plot to kill the President and whoever is going to do it has someone in the Secret Service. He believes this is what the agent, who was killed wanted to talk to Pete about. So an investigation is ordered and another agent, David Breckenridge, Pete's protégé is placed in charged. All agents are subjected to lie detector tests and Garrison fails but not because he's the one, but because he's been having an affair with the First Lady. Later more evidence is unearthed that incriminates Garrison even further which he says was planted. Breckenridge refuses to believe him because of some personal issues they have. So Garrison escapes and tries to find his informant, who's disappeared.