The seamy Secret Service prostitution scandal was at centre stage in the U.S. capital on Wednesday as the head of the agency in charge of protecting the president made a high-profile apology to kick off a congressional hearing.

Mark Sullivan, director of the Secret Service, took the stand at a packed U.S. Senate hearing into the scandal and apologized "for the conduct of these employees and the distraction it has caused."

His mea culpa came a month after some of his agents, several of them married, were caught hiring Colombian hookers in advance of President Barack Obama's visit to the country for a South American summit.

Prostitution is legal in Colombia, but soon after the scandal, Sullivan put in place new guidelines specifying that Secret Service agents on assignment abroad are subject to American law.

Wednesday's hearing was expected to reveal further salacious details in a scandal that has transfixed Americans since it came to light when a Secret Service agent balked at the fee charged to him by a prostitute at a Cartagena hotel on April 12.

64 allegations of sexual misconduct

Senator Joseph Lieberman, chairman of the Senate Homeland Security and Governmental Affairs Committee, told the hearing that there have been 64 allegations or complaints of sexual misconduct made against Secret Service employees in the last five years, including one of "non-consensual intercourse."

Lieberman, an independent, said the statistics raise questions about the culture of the agency tasked with protecting the president and those close to him. Senator Susan Collins agreed.

"This was not a one-time event," said Collins, the senior Republican on the committee. "The circumstances unfortunately suggest an issue of culture."

She took issue with suggestions in the early days of the scandal that it appeared to be an isolated incident.

Two of those involved, she pointed out, had been Secret Service employees for more than 20 years. The involvement of those married supervisors, who have since lost their jobs, sent "a message to the rank and file that this kind of activity is tolerated," Collins said.

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U.S. President Barack Obama walks with Secret Service agents in Florida. Secret Service Director Mark Sullivan will be grilled today in Washington at a congressional hearing looking into a prostitution scandal involving his agents in Colombia. (Kevin Lamarque/Reuters)

A Secret Service contingent was in Cartagena, staying in two separate hotels, and went out to different nightspots but "all ended up in similar circumstances," Collins said.

"Contrary to the conventional story line, this was not simply a single, organized group that went out for a night on the town together," Collins said.

Lieberman was equally skeptical.

"It is hard for many people, including me I will admit, to believe that on one night in April 2012, in Cartagena, Colombia, 12 Secret Service agents — there to protect the president — suddenly and spontaneously did something they or other agents had never done before," he said.

"That is to say, gone in groups of two, three or four to four different nightclubs or strip clubs and drink to excess and bring foreign national women back to their hotel rooms."

8 agents lost their jobs

A dozen Secret Service officers and supervisors, and another 12 U.S. military staff, have been embroiled in the scandal. Eight Secret Service agents have lost their jobs.

The agency is also attempting to permanently revoke the security clearance for another employee, while three others have been cleared of serious wrongdoing.

And yet the story continues to get more sordid.

The Washington Post reported this week that four of the fired agents are fighting their dismissals. The agents argue that they're being made scapegoats for behaviour that was known and tolerated for years.

"Current and former agency employees say sexual encounters during official travel had been condoned under an unwritten code that allows what happens on the road to stay there," the Post reported.

Sullivan was asked about the Post story on Wednesday, but said it was difficult for the Secret Service to investigate allegations made by anonymous sources.

Blackmail opportunities

In his opening remarks, he insisted that what transpired in Colombia was hardly typical of the Secret Service's nearly 7,000 employees — "the most dedicated, hardest working, self-sacrificing employees within the federal government."

"I can understand how the question could be asked," he allowed.

Collins pointed out the prostitution scandal could have handed blackmail opportunities to foreign intelligence agents, drug cartels or other criminals — something that could have ultimately placed the president in danger.

Sullivan denied that too, assuring the committee that Obama's security was never at risk in Colombia as agents cavorted with prostitutes. He ruled out the possibility of pillow talk, saying there was no way the agents could have disclosed closely guarded security information because they had not yet been briefed about the logistics of the presidential visit.

"At the time the misconduct occurred, none of the individuals involved in the misconduct had received any specific protective information, sensitive security documents, firearms, radios or other security-related equipment in their hotel rooms," Sullivan said.