WASHINGTON - The director of the US Secret Service publicly apologized for the first time Wednesday for a prostitution scandal that has rocked his agency as senior lawmakers strongly disputed his insistence that what unfolded last month in Cartagena, Colombia, occurred in isolation.

Part of the skepticism shared by lawmakers stemmed from fresh information they shared at the hearing, including allegations against Secret Service employees regarding nonconsensual sex, soliciting prostitutes on the streets of Washington, and hotel parties with underage women during the 2002 Salt Lake City Olympics.

Mark Sullivan, who has enjoyed strong bipartisan congressional support in the weeks since the scandal, told a Senate panel that “I am deeply disappointed, and I apologize for the misconduct of these employees and the distraction that it has caused.’’ Putting it more bluntly later, Sullivan said the employees involved “did some really dumb things.’’

But under questioning, Sullivan refused to submit that the mid-April incident is part of a broader agency culture that condones heavy drinking, partying, and sex during the off-hours of security assignments.

He also dismissed as “absurd’’ reports by the Washington Post that tolerance for inappropriate conduct is part of a culture that some employees call the “Secret Circus.’’

But members of the Senate Homeland Security and Governmental Affairs Committee said that evidence suggests that the scandal that unfolded in the hours before President Obama arrived in Cartagena for a summit is part of a pattern.

Sullivan “has a difficult time coming to grips with the fact that he has a broader problem than just this one incident,’’ Senator Susan Collins, Republican of Maine, the panel’s ranking Republican, said after the hearing.

“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.

Committee chairman Joseph Lieberman, a Connecticut independent, said that “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.’’

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

Lieberman pressed Sullivan and Homeland Security Acting Inspector General Charles Edwards during the hearing for details of 64 misconduct allegations against the Secret Service submitted in the last five years.

In one case, Lieberman said an off-duty uniformed Secret Service officer driving an agency vehicle in 2008 attempted to pick up an undercover police officer posing as a prostitute. Sullivan told the committee that the officer was “separated from the agency’’ a month later.

Another case involving allegations of nonconsensual sex against an agent was not pursued by law enforcement officials who investigated the matter, Sullivan said.

Edwards also 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 2002 Winter Olympics.

Sullivan said the agency investigated the case when it happened and disciplined the employees involved. At least one of the employees involved resigned; another was fired, according to congressional aides familiar with the case.

Lieberman said other misconduct cases involved employees who sent sexually explicit e-mails or material on government computers and at least 30 cases involving alcohol or charges of driving while under the influence.

During the hearing Edwards said he is launching a separate, independent investigation that would, among other things, involve interviews with the 12 employees implicated in the scandal. The new probe also will review whether Sullivan’s internal investigation was rushed or whether the move to oust most of the men involved in the scandal was proper, according to legal officials familiar with the case, who asked not to be named because of the nature of the ongoing probe.

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 alleged misconduct probes.

Edwards told lawmakers Wednesday that he received his first briefing on the situation on April 13, the day that 12 agency employees were sent home from Colombia.

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 also plans to review how many and what types of polygraph tests were conducted on agency personnel.