Napolitano's appearance before the Senate Judiciary Committee was her first public comment on the scandal. She called drinking and consorting with prostitutes by agents sent to Cartagena, Colombia, to prepare for President Obama's trip there "inexcusable," the Los Angeles Times reported.
"Part of our investigation is confirming that this was an aberration -- or not," Napolitano said.
She told the committee investigators from the Office of Professional Responsibility have found no indication of similar conduct in more than two years of records, Politico said.
Twelve agents were caught up in the scandal. Eight have quit under pressure or face dismissal, three could face internal discipline, although they have been cleared of major misconduct, and one has lost his security clearance, which would bar him from the Secret Service.
Napolitano said the agency generally "does a marvelous job." She also said Secret Service Director Mark Sullivan has President Obama's support.
Sources told The Washington Post Tuesday some of the agents who lost their jobs for allegedly partying with prostitutes in Colombia may argue hard partying during overseas trips is common practice. One agent not directly involved said the trip to Cartagena only became a scandal because U.S. diplomats got involved.
"Of course it has happened before," the agent said. "This is not the first time."
Obama, asked about the episode Tuesday by late-night TV host Jimmy Fallon, praised the Secret Service and said its reputation should not be tarnished by "a couple of knuckleheads."