The U.S. Secret Service says three more agents have been forced out of their jobs because of a prostitution scandal that has shaken the agency that protects the president.

Officials said Tuesday two agents resigned and a third had his security clearance revoked. Two others were cleared of charges of serious misconduct.

This brings to nine the number of Secret Service agents who have lost their jobs for consorting with prostitutes in Cartagena, Colombia earlier this month — days before President Barack Obama arrived for the Summit of the Americas. The Pentagon is also investigating 12 military members who were allegedly involved.

During the taping of his appearance on an NBC television talk show Tuesday, Mr. Obama called the agents caught in the scandal “knuckleheads.” But he said they should not detract from what the Secret Service does. The president called the majority of the agents incredible guys, protecting him and his family as well as U.S. officials all over the world.

Prostitution is legal in Colombia, but off-limits for many U.S. government employees because of the possible security risks.