The director of the U.S. Secret Service publicly apologized for the first time Wednesday for a prostitution scandal that has rocked his agency, insisting that what occurred in Colombia last month did not jeopardize President Obama’s security and was an isolated case.

But senior lawmakers voiced deep skepticism that the Secret Service has gotten to the bottom of its problems, calling for a more comprehensive inquiry into the culture of the 7,000-member agency.

In briefings in recent weeks, lawmakers said they learned details about other incidents — including a case in Washington in 2008 in which a uniformed Secret Service officer driving an agency vehicle attempted to pick up an undercover police officer posing as a prostitute.

After listening to testimony from Secret Service Director Mark Sullivan, who offered profuse apologies and acknowledged that his employees did “some really dumb things” on the trip to Cartagena, the top-ranking Republican lawmaker on the Senate Homeland Security and Governmental Affairs Committee said Sullivan does not understand the scope of his agency’s dysfunction.

Sullivan “has a difficult time coming to grips with the fact that he has a broader problem than just this one incident,” said Sen. Susan Collins (Maine).

“He kept saying over and over again that he basically does think this is an isolated incident, and I don’t think he has any basis for that conclusion,” she added.

Twelve Secret Service employees were implicated in the events in Cartagena after supervisors learned they had brought local women back to their hotel rooms. At least eight employees were forced to resign or retire, but at least four are now fighting their dismissals.

Several agents said that sexual encounters with women abroad are routine and that management condones them.

At the hearing Wednesday, Sullivan disputed that notion, denying under questioning that the situation in mid-April was part of a larger pattern of heavy drinking and sexual encounters during presidential trips. He also dismissed as “absurd” reports by The Washington Post that tolerance of inappropriate conduct is part of a culture that some employees call the “Secret Circus.”

“I’ve worked for a lot of men and women in this organization, and I never one time had any supervisor or any other agent tell me that this type of behavior is condoned,” Sullivan told the panel. “I know I’ve never told any of our employees that it’s condoned.”

At the same hearing, however, the acting inspector general of the Department of Homeland Security — the umbrella agency that includes the Secret Service — said his inquiry will expand to not just review how the agency investigated itself but also the broader culture of the agency.

Members of the committee described an alarming pattern of misconduct at the agency that had been reported over the years.

Chairman Joseph I. Lieberman (I-Conn.) pressed Sullivan and Inspector General Charles K. Edwards for details of 64 misconduct allegations made against the Secret Service in the past five years. They include allegations against employees regarding non-consensual sex and soliciting prostitutes. An earlier incident involved hotel parties with underage girls during the 2002 Salt Lake City Olympics.

“For the good of the Secret Service, as he decides how to change the rules and procedures of the Secret Service, he has got to assume that what happened in Cartagena was not an isolated incident, or else it will happen again,” Lieberman said of Sullivan after the hearing.

Regarding the prostitution case in Washington, which Lieberman cited as an example of misbehavior, Sullivan told the committee that the officer was “separated from the agency” a month later.

Another case involving allegations against an agent of non-consensual sex was not pursued by law enforcement officials who looked into the matter, Sullivan said.

Despite their skepticism, Lieberman, Collins and other senators said Sullivan should remain as director.

Sullivan, who has received strong bipartisan congressional support in the weeks since the scandal, told the lawmakers that he is “deeply disappointed” and apologized for “the misconduct of these employees and the distraction that it has caused.”

Edwards, the acting inspector general at DHS, said he is reviewing new information from a case involving at least three Secret Service employees who attended an alcohol-infused party with underage girls in a Salt Lake City hotel room during the Winter Olympics.

That case, Sullivan said, was investigated at the time, and the employees were disciplined. At least one of them resigned and another was fired, said congressional aides familiar with the case.

Lieberman said that at least three separate misconduct cases involved employee interactions with foreigners. Other incidents involved employees who sent sexually explicit e-mails or material on government computers and some alcohol-related events that included charges of driving while under the influence.

During the hearing, Edwards said he is opening a separate, independent investigation that would, among other things, involve interviews with the 12 employees implicated in the Colombia scandal. The new inquiry will review whether Sullivan’s internal investigation was rushed or whether the move to oust most of the men involved was proper, according to legal officials familiar with the case, who spoke on the condition of anonymity because of the nature of the inquiry.

The officials said Edwards plans to examine allegations that the Secret Service rushed to judgment and handled its questioning of the men in the prostitution scandal differently than in other investigations of alleged misconduct.

Edwards told lawmakers on Wednesday that he received his first briefing on the situation on April 13, the day that the 12 Secret Service employees were returned to the United States. His office plans to review notes from interviews with nearly 200 agency employees who were in Colombia and 25 employees at the hotels in Cartagena. He plans to review how many polygraph tests and what types were conducted on agency personnel.

Edwards said Sullivan “has repeatedly stated to me his commitment to conduct a complete and thorough investigation. His actions so far have demonstrated that commitment.”

Staff writers Carol D. Leonnig and David Nakamura contributed to this report.