Several small groups of U.S. Secret Service employees separately visited clubs, bars and brothels in Colombia prior to a visit by President Barack Obama last month and engaged in reckless, "morally repugnant" behaviour, Senator Susan Collins says.

She says the employees' actions during the stunning prostitution scandal could have provided a foreign intelligence service, drug cartels or other criminals with opportunities for blackmail or coercion that could have threatened the president's safety.

In remarks prepared for the first congressional hearing on the matter which begins today at 10:30 a.m. ET, Collins, a Republican from Maine, also challenged early assurances that the scandal in Colombia appeared to be an isolated incident. She noted that two participants were Secret Service supervisors — one with 21 years of service and the other with 22 years — and both were married. Their involvement "surely sends a message to the rank and file that this kind of activity is tolerated on the road," Collins said.

'This was not a one-time event. The circumstances unfortunately suggest an issue of culture.'—Susan Collins, Republican senator

"This was not a one-time event," said Collins, the senior Republican on the Senate Homeland Security and Governmental Affairs Committee. "The circumstances unfortunately suggest an issue of culture."

Independent Connecticut Senator Joe Lieberman, the committee's chairman, said, "I want to hear what the Secret Service is doing to encourage people to report egregious behaviour when they see it."

He also said he want to know what is being done to ensure a similar incident does not happen in the future.

Charles Edwards, the acting inspector general of the Department of Homeland Security, will also attend the hearing.

New details may be exposed

Wednesday's hearing was expected to expose new details in the scandal, which became public after a dispute over payment between a Secret Service agent and a prostitute at a Cartagena hotel on April 12. The Secret Service was in the coastal resort for a Latin American summit before Obama's arrival. Collins said several small groups of agency employees from two hotels went out separately to clubs, bars and brothels and they "all ended up in similar circumstances."

"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.

Senators were expected to focus on whether the Secret Service permitted a culture in which such behaviour was tolerated. Homeland Security Secretary Janet Napolitano has testified previously that she would be surprised if there were other examples, but senators have been skeptical.

In his own prepared remarks, Secret Service Director Mark Sullivan told senators the behaviour in Colombia wasn't representative of the agency's nearly 7,000 employees.

"I can understand how the question could be asked," Sullivan said, calling his employees "among the most dedicated, hardest working, self-sacrificing employees within the federal government."

Obama never at risk, agency maintains

Sullivan also assured senators that Obama's security was never at risk. The officers implicated in the prostitution scandal could not have inadvertently disclosed sensitive security details because their confidential briefing about Obama's trip had not taken place.

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)

"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.

Sullivan has survived professionally so far based on his openness about what happened. Senators were not expected to ask for his resignation, and the acting inspector general for the Homeland Security Department, Charles K. Edwards, gave Sullivan high marks for integrity. Edwards, who estimated that the early stages of his own investigation would be finished before July 2, said the Secret Service "has been completely transparent and co-operative."

"The Secret Service's efforts to date in investigating its own employees should not be discounted," Edwards told senators. "It has done credible job of uncovering the facts and, where appropriate, it has taken swift and decisive action."

The White House on Tuesday reasserted its confidence in Sullivan. Obama "has great faith in the Secret Service, believes the director has done an excellent job," White House spokesman Jay Carney said. "The director moved very quickly to have this matter investigated and took action very quickly as a result of that investigation."

A dozen Secret Service officers and supervisors and 12 other U.S. military personnel were implicated. Eight Secret Service employees, including the two supervisors, have lost their jobs. The Secret Service is moving to permanently revoke the security clearance for one other employee, and three others have been cleared of serious wrongdoing.

Fired officers fight dismissals

The Washington Post reported Tuesday that four of the Secret Service employees have decided to fight their dismissals.

Prostitution is legal in Colombia, but Sullivan quickly issued new guidelines that made it clear that agents on assignment overseas are subject to U.S. laws.

Sullivan said he directed Secret Service inspectors to investigate reports of similar misconduct in San Salvador. After 28 interviews with hotel employees and managers, State Department officials and others, "no evidence was found to substantiate the allegations," Sullivan said.

This week the Drug Enforcement Administration said the Justice Department's Office of the Inspector General was investigating possible misconduct by two or more agents in Colombia. Collins revealed that the case involved at least two DEA employees who entertained female masseuses in the Cartagena apartment of one of the DEA agents. The investigation is unrelated to the Secret Service scandal but is based on information provided to the DEA by the Secret Service.