Sullivan said at a U.S. Homeland Security and Governmental Affairs Committee hearing the 12 agents had brought women to their hotel rooms before being briefed on details of the plan to protect President Barack Obama during his visit to Cartagena for a summit meeting.
"During the course of the investigation, we confirmed that none of the 12 individuals had received a briefing regarding their protective assignment prior to the misconduct taking place," Sullivan said. "We also confirmed that none of the 12 individuals had any sensitive security documents, firearms, radios or other security-related equipment in their hotel room."
Sullivan said the agents' alleged misconduct "is not representative ... of the high ethical standards we demand from our nearly 7,000 employees. I am deeply disappointed, and I apologize for the misconduct of these employees and the distraction that it has caused."
While responding to questions, Sullivan said he was "dumbfounded" when he learned of the agents' alleged misconduct.
The Secret Service chief said the agency has "zero tolerance for this type of behavior."
Sen. Joseph Lieberman, I-Conn., said: "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, which is gone out in groups of two, three or four to four different nightclubs or strip clubs, drink to excess and then bring foreign national women back to their hotel rooms."
"This was not a case where these 12 men together were out on the town in the same club, bringing back women from that one source," said committee member Sen. Susan Collins, R-Maine. "They went out on the town in small groups -- in some cases, two or three or individually -- yet each one of them comes back to the hotel, makes no attempt to conceal the fact that they're bringing foreign nationals into the hotel, actually register them at the front desk. They don't try to conceal their actions in any way.
"And that suggests to me that they weren't worried about being caught, that they did not think there would be consequences if they were caught," Collin said. "Otherwise, wouldn't you expect that they would try conceal their actions?"
Meanwhile, four Secret Service agents said they will fight their dismissals for engaging in improper conduct in Colombia, saying they didn't break the rules.
The agents argue the agency made them scapegoats for behavior that has been long-tolerated in the Secret Service, The Washington Post reported.
The incident last month before Obama's trip to the Summit of the Americas in Colombia involved Secret Service and U.S. military members who allegedly hired prostitutes.
Under questioning Wednesday, Sullivan said a Secret Service employee "was separated" from the agency after being involved in prostitution in 2008.