Senators on Capitol Hill challenged Napolitano to reconcile the image of agents who protect the lives of America's president with the dozen officers and supervisors implicated in the sordid affair.
Napolitano stressed that her agency's investigation will get to the bottom of the embarrassing scandal, saying they "will leave no stone unturned." So far, nine of the 12 Secret Service employees implicated in the scandal have been forced out of the agency.
"The director took immediate action to remove the agents involved and a full and thorough investigation is under way to determine exactly what transpired and actions we need to take to ensure this kind of conduct doesn't happen again," she said.
Napolitano said the agency's office of professional responsibility had never received previous complaints in the past 2 1/2 years, but it was unclear why she specified that period.
The Defense Department acknowledged that it knew that six military personnel had broken curfew rules prior to President Barack Obama's arrival at a Latin American summit but let them remain on the job.
The chairman of the judiciary committee, Sen. Patrick Leahy, D-Vt., praised the Secret Service as "wise, very professional men and women," and called it shocking that so many of the agency's employees were involved in the scandal.
"It really was, I think, a huge disappointment to the men and women of the Secret Service to begin with, who uphold very high standards and who feel their own reputations are now besmirched by the actions of a few," Napolitano said.
The judiciary committee hearing was the first time Napolitano faced public questioning from lawmakers since the scandal.
The Associated Press contributed to this report.