US Secret Service Director Mark Sullivan, in his first congressional appearance since the prostitution scandal erupted in April, on Wednesday rejected allegations that a culture of improper behavior exists in his agency.

"This is not a cultural issue, this is not a systemic issue," Mr. Sullivan told the Senate Homeland Security Committee.

But lawmakers’ questions at the hearing showed the Service is struggling to shake off the scandal involving prostitutes in Cartagena, Colombia.

The committee chairman, Sen. Joe Lieberman (I) of Connecticut, reported that there have been 64 allegations of misconduct by agents over the past five years, including improper relationships with foreign nationals.

Some former employees of the agency say that it is especially on assignments abroad that the culture of improper behavior exists.

Although Sullivan enjoys a good reputation with Congress, some lawmakers remain skeptical of his defense of his agency.

"The facts so far lead me to conclude that, while not at all representative of the majority of Secret Service personnel, this misconduct was almost certainly not an isolated incident," Sen. Susan Collins, (R) of Maine, said. "The numbers [of agents] involved, as well as the participation of two senior supervisors, lead me to believe that this was not a one-time event. Rather, it suggests an issue of culture."

"Two of the participants were supervisors – one with 22 years of service and the other with 21 – and both were married,” Senator Collins pointed out. “That surely sends a message to the rank and file that this kind of activity is tolerated on the road.”

In any case, and under pressure not to have the Secret Service simply investigate itself, the Department of Homeland Security’s inspector general said Wednesday that an independent probe of the Cartagena incident will be conducted.

The agents alleged to have been involved in Cartagena were part of an advance team sent to prepare for President Obama’s participation in the Summit of the Americas. Following exposure of the incident, which included some 20 paid female “escorts,” the number of agents alleged to be involved grew to 13, eight of whom were either fired, forced to retire, or had their security clearances pulled (which would force them to resign).

Although the incident was serious and certainly an embarrassment to an agency charged with protecting the president and other high-ranking US officials, Sullivan said there had been no security breaches in Colombia.

"At the time the misconduct occurred, none of the individuals involved … had received any specific protective information, sensitive security documents, firearms, radios or other security related equipment in their hotel rooms," Sullivan said in his testimony. The officers had not yet received their briefing on Obama's attendance at the summit.

Still, he said, “I am deeply disappointed, and I apologize for the misconduct of these employees and the distraction that it has caused.”

As the scandal unfolded last month, Sullivan – a 29-year veteran of the Secret Service whose duty assignments include two tours with the Presidential Protective Division – responded with a stern note to agents about their behavior, especially when on assignment abroad.

In an internal memo Sullivan said, “Alcohol may only be consumed in moderate amounts” and drinking less than 10 hours before reporting for duty is prohibited. (Previous rules included a six-hour limit between drinking and work.)

Most important in light of the recent episode in Cartagena, foreign nationals other than hotel staff are not allowed in agents’ rooms, and agents are not allowed to patronize “nonreputable establishments” (strip clubs and brothels). The fact that prostitution is legal in some parts of Colombia makes no difference, he implied.

"The absence of a specific, published standard of conduct covering an act or behavior does not mean that the act is condoned, is permissible or will not call for – and result in – corrective or disciplinary action,” Sullivan warned. "All employees have a continuing obligation to confront expected abuses or perceived misconduct.”

The Washington Post reported Tuesday that “four Secret Service employees have decided to fight their dismissals … a development that could unravel what has been a swift and tidy resolution to an embarrassing scandal over agents’ hiring of prostitutes.”

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“The agents are arguing that the agency is making them scapegoats for behavior that the Secret Service has long tolerated,” the newspaper reported, a charge that Director Sullivan in his Senate testimony Wednesday called “just absurd.”

In addition to the Secret Service scandal, the US military is investigating the involvement of 12 enlisted personnel on assignment in Colombia as part of the security detail last month. The Drug Enforcement Administration also is investigating reports that DEA agents posted to Colombia have brought prostitutes to their apartments.