"The U.S. Secret Service has a history of executing its mission with professionalism, honor and integrity, and Director Sullivan's six-year stewardship of the agency has been marked by these traits," Napolitano wrote. "In the aftermath of allegations of personnel misconduct in Colombia, Director Sullivan took immediate and decisive action to remove the agents involved, investigate what transpired and ensure the Secret Service continued performing their vital protection mission. I have the highest confidence in the Director's leadership and the utmost respect for his distinguished 34 years of law enforcement service."
Hotel Worker Suspected Drug Use
In addition to the inquiry about prostitutes and other misconduct, Secret Service investigators are probing reports from a Cartagena, Colombia, hotel worker, who said he saw a line of white powder, which he believed to be cocaine, on a table in a Secret Service agent's room.
The hotel worker told the New York Post he responded to clean up the room after there was a dispute between a Secret Service agent and a prostitute over payment.
"When I went upstairs, I walked into a messy room. The room was littered with two whiskey bottles -- and a line of white powder, I believed to be cocaine, was on top of a round glass table in the room," the staffer told the Post.
According to Rep. Peter King, R-N.Y., who was briefed by the Secret Service, the agency is taking the hotel worker's allegations seriously.
"This is one of the things the Secret Service is investigating," King told ABC News. "Agents are randomly tested for drugs. I know the director will take further action if more information on this becomes available."
The Secret Service declined to comment on the record, but sources familiar with the investigation said inspectors in Colombia have yet to be told directly of the information in the New York Post report. However, sources said the agency will follow up anyway and question agents who travelled to Colombia about possible drug use.
According the New York Post story, the hotel worker described a chaotic, morning-after scene in the hotel lobby, with the prostitute screaming in the lobby that she had not been paid.
The worker said, "The agent was supposed to pay her a [bar] fine on top of the pay rate for her sexual services, but he didn't."
The worker explained that visitors to area strip clubs are expected to pay a fee to the club, and then pay the woman directly for any sexual services.
As part of the prostitution probe, agents have agreed to polygraph tests. It wasn't clear whether drug questions would be included.
ABC News' Jake Tapper and Michael S. James contributed to this report.