A supervisor caught in the U.S. Secret Service prostitute scandal has agreed to retire, officials said Wednesday.

Another agent has said he will resign, The Washington Post reported. A total of 11 agents, two of them supervisors, are under investigation for allegedly partying with prostitutes -- along with members of the military -- while preparing for President Barack Obama's trip last week to Cartagena, Colombia.

The agency has recommended the other supervisor for dismissal, officials said. He would be able to appeal.

Sen. Susan Collins, R-Maine, said the women the agents allegedly brought to a Cartagena hotel might have been be spies.

"We don't know who these women are," she told reporters in Washington. "They could be spies. They could be associated with hostile forces. They could have disabled the agents' weapons or planted listening devices or in other ways breach security. It is very serious."

She said investigators suspect the agents and military personnel brought as many as 21 prostitutes to the sprawling beachfront Hotel Caribe in Cartagena, Colombia, April 11 during a night of carousing before Obama's April 13 arrival for the sixth Summit of the Americas.

Some of the "20 to 21" women accompanied Secret Service agents, and others escorted members of the military, which is conducting its own investigation, said Collins, the senior Republican on the Senate Homeland Security Committee.

Collins said she was briefed on the matter by Secret Service Director Mark Sullivan.

House Homeland Security Committee Chairman Peter King, R-N.Y., also briefed by Sullivan, said Tuesday interviews by Secret Service investigators in Colombia would help address concerns any of the women had ties to organized crime or drug cartels.

The youngest of the women was 20 years old, he said.

The agents had not yet been briefed on their specific assignments or started their official duties when they went out and met the women, King said.

They have "different recollections about what happened, or are not telling the truth," he said.

Some officers "are claiming [the women] aren't prostitutes, just women they met in a bar," King was quoted by The Wall Street Journal as saying.

That wouldn't necessarily resolve the concerns, he said, because "Secret Service shouldn't be bringing outside people into a secure zone."

White House spokesman Jay Carney said Obama had "confidence" in Sullivan -- appointed to his post by President George W. Bush in 2006 -- and would await the results of the agency's investigation before weighing in further.

"The director acted swiftly in response to this incident and is overseeing an investigation that obviously needs to be conducted," Carney said, adding the administration would not "speculate about conclusions it might reach."

Topics: Susan Collins, Mark Sullivan, Peter King, Barack Obama, Jay Carney, George W. Bush