Secret Service Director Mark Sullivan, facing questions on Capitol Hill about whether the escapades could have jeopardized the president's security, said he had referred the matter to an independent government investigator.
Sullivan said the 11 Secret Service agents and 10 military personnel under investigation were telling different stories about who the women were. Sullivan has dispatched more investigators to Colombia to interview the women, said Rep. Peter King, R-N.Y., chairman of the House Homeland Security Committee.
"Some are admitting (the women) were prostitutes, others are saying they're not, they're just women they met at the hotel bar," King said in a telephone interview. Sullivan said none of the women, who had to surrender their IDs at the hotel, were minors. "But prostitutes or not, to be bringing a foreign national back into a secure zone is a problem."
Sen. Susan Collins of Maine, the ranking Republican on the Homeland Security Committee, said Tuesday that "20 or 21 women foreign nationals" were brought to the hotel. Eleven of the Americans involved were Secret Service, she said, and "allegedly Marines were involved with the rest."
"This could have been disastrous," King said.
The growing scandal has become an election-year embarrassment for Obama, who has said he would be angry if the allegations proved to be true. The White House said Obama had confidence in the Secret Service chief.
"Director Sullivan acted quickly in response to this incident and is overseeing an investigation as we speak into the matter," White House press secretary Jay Carney said.
At least 10 U.S. military personnel staying at the same hotel were also being investigated for their role in the alleged misconduct.
Two U.S. military officials said they include five Army Green Berets. Both officials spoke on condition of anonymity about an investigation that is still under way.
One of the officials said the group also includes two Navy Explosive Ordinance Disposal technicians, two Marine dog handlers and an Air Force airman. The Special Forces Green Berets were working with Colombia's counterterrorist teams, the official said.
The Secret Service did not immediately respond to a request for comment about Grassley's account.