It was the first time the head of the US Secret Service had appeared before congress to apologise to lawmakers for the scandal involving his employees and Colombian prostitutes.

Still it seems US politicians are not quite ready for forgiveness.

As Mark Sullivan, the director of the US Secret Service, appeared before the Homeland Security and Governmental Affairs Committee on Wednesday, he expressed regret to lawmakers for the behaviour of his employees, entrusted with protecting the US president.

"I am deeply disappointed and I apologise for the misconduct of these employees and the distraction it has caused," Sullivan said.  

The Secret Service prostitution scandal broke last month, when it was learned 12 officers and agents, while preparing for President Obama's arrival in Cartagena, Colombia, spent their off-duty hours drinking heavily and even bringing prostitutes back to their hotel rooms.

Immediately Sullivan said he ordered an investigation into the incident and removed the officers involved from their duties.

Senator sceptical

Sullivan has reportedly also set up a "professional reinforcement working group" to review the agency's standards of conduct.

Still, US Senator Joseph Lieberman said he believes the scandal was not an isolated incident inside the Secret Service and is calling for further investigation.
  
"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," Lieberman said. 

Yet that's exactly what the Secret Service director repeatedly asked of senators.

He said there was never sensitive information in the hotel rooms and the president was never in danger. 

Sullivan called the actions of the 12 men accused of misconduct "dumb", but he said their behaviour was not systemic within the Secret Service.  

"The thought or belief that this behaviour is condoned is absurd," he told senators.

'High ethical standards'

Sullivan said the agents' actions are "not representative of [the] values or the high ethical standards we demand".

The congressional committee overseeing the Secret Service investigation appeared unsatisfied.

It accused agents of doing little to conceal their actions from supervisors, when they used their real names to sign in the prostitutes they brought to their hotel.

Senator Susan Collins claimed this demonstrates a culture of tolerance within the agency for what she called "repugnant" behaviour.

"This misconduct was almost certainly not an isolated incident," Collins said.

The committee reports 64 additional allegations in the past five years of sexual misconduct against Secret Service employees have surfaced since the prostitution scandal broke.

An inspector-general has been called in to conduct an independent review of the Secret Service's investigation.

Its first report is expected in July.