CHAPEL HILL, North Carolina (Reuters) - President Barack Obama on Tuesday blamed a Colombia prostitution scandal engulfing the U.S. Secret Service on the misconduct of a "couple of knuckleheads" and insisted that the vast majority of agents perform their work admirably.
Obama's comments appeared to play down the extent of the controversy, the worst in decades to hit the agency responsible for protecting the president, his family and other senior officials.
"What these guys were thinking, I don't know. That's why they're not there anymore," Obama said during a taping of an appearance on NBC's "Late Night with Jimmy Fallon" program while on a visit to North Carolina.
At the same time, there are signs of a widening scope of the inquiries into allegations that agents took prostitutes back to their hotel rooms in Cartagena earlier this month on the eve of Obama's arrival for a hemispheric summit.
Joseph Lieberman, chairman of the Senate Homeland Security Committee, said "whistleblower people" had called his office with allegations about past misconduct by Secret Service personnel and "we're beginning to talk to them."
The scandal, the latest election-year headache for Obama, has raised questions whether the Secret Service has a deeper cultural problem, even as the White House has insisted that president retains confidence in its director, Mark Sullivan.
Six Secret Service employees have left their jobs after being implicated in a night of partying and carousing that embarrassed the U.S. government and overshadowed Obama's participation in the Summit of the Americas.
Six more Secret Service members and 12 U.S. military personnel remain under investigation. The men are alleged to have taken as many as 21 prostitutes back to their beachfront hotel in Cartagena on the night of April 11-12.
Obama, who has said he would be "angry" if the accusations are proved true, addressed the matter in public for only the second time on Tuesday and was quick to defend the Secret Service's reputation.
He said "99.9 percent" of Secret Service agents put their lives on the line and did a great job. "These guys are incredible. They protect me, they protect Michelle, they protect our girls, they protect our officials all around the world," Obama said.
"A couple of knuckleheads shouldn't detract from, you know, what they do," he said.
Despite that, Lieberman plans hearings on the scandal in the near future. "Since we announced the investigation, some whistleblower people have called the office. So we're beginning to talk to them. When we're ready, we'll go public," Lieberman, an independent, told reporters outside the Senate.
"Some of it seems credible, or at least worth investigating more," he said. A committee aide said just one call had been received so far.
Lieberman said the focus of his committee's probe would not be events in Cartagena, which is being investigated by the Secret Service, but any incidents in recent years.
"I want to ask questions about whether there was any other evidence of misconduct by Secret Service agents in the last five or 10 years," he said. "If so, what was done about it? Could something have been done to have prevented what happened in Cartagena? And now that it has happened, what do they intend to do?"
The leaders of the Senate Armed Services Committee, Carl Levin and John McCain, said they expected to get a briefing on Wednesday from the Pentagon on the part played by military service members in the scandal.
McCain, a Republican, complained that they should have been briefed sooner, noting that Sullivan had been briefing lawmakers from other committees that oversee that agency.
"Secretary Panetta doesn't, I think, sufficiently understand the role of the Armed Services Committee," McCain said.
(The story has been refiled to add dropped word in paragraph 4)
Additional reporting by Susan Cornwell; Writing by Matt Spetalnick