detector tests in prostitution scandal NBC News' Mark Potter traces the events in the unfolding Secret Service scandal. Some of the Secret Service agents under investigation in the Colombian prostitution scandal have agreed to take polygraph tests, a U.S. official told NBC News on Wednesday. Libby Leist of NBC News contributed to this report by Kristen Welker of NBC News and M. Alex Johnson of msnbc.com. Eleven Secret Service agents were recalled from Colombia last week and have been stripped of their security clearances after reports emerged alleging that some of them had taken prostitutes to their hotel rooms before President Barack Obama arrived for the Latin American summit. The U.S. official said the agents had been "offered" the opportunity to submit to polygraph tests and that some had accepted. The official didn't say how many had agreed. Ed Donovan, a spokesman for the Secret Service, wouldn't confirm the information, saying only that the agency's Office of Professional Responsibility was using every investigative tool at its disposal. NBC News: Prostitute's $50 fee for two agents triggered Secret Service scandal Secret Service Director Mark Sullivan has told lawmakers that the 11 agents and 10 U.S. military personnel also implicated in the scandal are giving investigators conflicting stories, making it difficult to pin down the truth, several lawmakers told NBC News. The Colombian government is separately investigating whether underage girls were part of the arrangements, but Sen. Charles Grassley of Iowa, the top Republican on the Judiciary Committee, told NBC News that Sullivan believes the youngest woman involved was about 20 or 21 years old. NBC News reported this week that some of the agents had copies of the president's schedule in their rooms, raising the possibility of a security breach. But Sullivan said none of the prostitutes ever had access to secure information, according to Grassley. "I think that he feels that protocol was followed," Grassley told NBC News. Grassley said Judiciary Committee staff members would meet with agency representatives later this week for a more complete briefing. He said the committee would conduct its own investigation only if members concluded that the Secret Service inquiry "was not doing the job." Regardless, Grassley said, "I think you'll find their heads are going to roll." He added that he was worried that there could be a culture of misbehavior at the Secret Service, a concern that was echoed by Sen. Susan Collins, R-Maine, who has also been briefed on the case. Collins told NBC News that her "instinct" is that this wasn't an isolated incident. She said that she pressed Sullivan and that he had told her the agency was "scrubbing the files" for possible previous incidents. The Defense Department is separately investigating the 10 military members who have been implicated. Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said Tuesday that "we let the boss down" in Colombia. Defense Secretary Leon Panetta is expected to brief leaders of the Senate Armed Services Committee in the next couple of days, Sen. John McCain, R-Ariz., told NBC News on Wednesday.