But Mark Sullivan's mea culpa didn't do much to convince senators that there isn't a culture of sexual bad behaviour at the Secret Service a month after some of his agents were caught hiring Colombian hookers in advance of President Barack Obama's visit to the country.
"For the good of the Secret Service, he's got to assume that what happened in Cartagena was not an isolated incident or else it will happen again," said Joe Lieberman, chairman of the Senate Homeland Security and Governmental Affairs Committee.
His colleague, Susan Collins, suggested Sullivan had his head in the sand.
"He kept saying over and over again that he basically does think this was an isolated incident and I don't think he has any basis for that conclusion," said Collins, the senior Republican on the committee.
Sullivan apologized "for the conduct of these employees and the distraction it has caused."
But he repeatedly denied suggestions that there is a cultural problem at the Secret Service that contributed to the scandal.
In Sullivan's opening remarks, he insisted that what transpired in Colombia was hardly typical of the Secret Service's nearly 7,000 employees — "the most dedicated, hardest working, self-sacrificing employees within the federal government."
"I can understand how the question could be asked," he allowed.
Prostitution is legal in Colombia, but soon after the scandal, Sullivan put in place new guidelines specifying that Secret Service agents on assignment abroad are subject to American law.
Wednesday's hearing revealed further details in a scandal that has rocked the Secret Service and transfixed Americans ever since an agent balked at the fee charged to him by a prostitute at a Cartagena hotel on April 12.
After the prostitute spent three hours pleading for him to pay the agreed-upon price, she complained to authorities, setting in motion a chain of events that soon resulted in the scandal making headlines around the world.
Lieberman told the hearing that there have been 64 allegations or complaints of sexual misconduct made against Secret Service employees in the last five years, including one of "non-consensual intercourse."
Other incidents included allegations of Secret Service agents partying with teenage girls at the 2002 Winter Olympics in Utah and an agent in D.C. purportedly soliciting sex from an undercover police officer.
Those sorts of allegations sound alarm bells about the culture of the agency tasked with protecting the president and those close to him, Lieberman said.
"This was not a one-time event," she said, describing the Colombia events as "morally repugnant." "The circumstances unfortunately suggest an issue of culture."
She took issue with suggestions in the early days of the scandal that it appeared to be an isolated incident.
Two of those involved, she pointed out, had been Secret Service employees for more than 20 years. The involvement of those married supervisors, who have since lost their jobs, sent "a message to the rank and file that this kind of activity is tolerated," Collins said.
Despite the Secret Service agents in Cartagena staying in two separate hotels and heading out to different nightspots, they "all ended up in similar circumstances," Collins said.
"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.
Lieberman echoed those sentiments.
"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," he said.
"That is to say, gone in groups of two, three or four to four different nightclubs or strip clubs and drink to excess and bring foreign national women back to their hotel rooms."
A dozen Secret Service officers and supervisors, and another 12 U.S. military staff, have been embroiled in the scandal that erupted just a day before Obama arrived in Colombia for the start of a South American summit. Eight Secret Service agents have lost their jobs.
The agency is also attempting to permanently nix the security clearance for another employee, while three others have been cleared of serious wrongdoing.
And yet the story continues to grow more sordid.
The Washington Post reported this week that four of the fired agents are fighting their dismissals. The agents argue that they're being made scapegoats for behaviour that was known and tolerated for years within the Secret Service.
"Current and former agency employees say sexual encounters during official travel had been condoned under an unwritten code that allows what happens on the road to stay there," the Post reported.
The unwritten code at the agency has reportedly long been nicknamed: "Wheels down, rings off."
Sullivan was asked about the Post story on Wednesday, and urged those making the allegations to come forward. He added, however, that any suggestion that the Secret Service condoned or authorized hiring prostitutes while abroad was "absurd."
Collins pointed out the prostitution scandal could have placed the president in danger since it might have handed blackmail opportunities to foreign intelligence agents, drug cartels or other criminals.
Sullivan denied that too, assuring the committee that Obama's security was never in jeopardy in Colombia as agents caroused with prostitutes.
He ruled out the possibility of pillow talk, saying there was no way the agents could have disclosed closely guarded security information because they had not yet been briefed about the logistics of the presidential visit.
"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.
The Secret Service head has long been respected on both sides of the aisle on Capitol Hill as an impeccably honest and diligent civil servant, likely explaining why he hasn't been forced to resign in the wake of the scandal.
Charles Edwards, the acting inspector general of homeland security, is conducting a separate investigation into the Colombian scandal.
Edwards told the hearing he planned to interview all of the agents implicated in the scandal, starting with two on Wednesday afternoon. He lauded Sullivan, a 29-year veteran of the Secret Service, and his agency for being "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."