EXCLUSIVE: Probe of Secret Service prostitution scandal may cite White House advance staff involvement Federal law enforcement personnel and a congressional committee are anxiously awaiting an overdue inspector general's report that they believe may reveal the involvement of two White House advance team members in the Secret Service prostitution scandal in Colombia earlier this year. While much of the attention in the case has focused on the actions of Secret Service personnel, multiple law enforcement and congressional sources tell FoxNews.com that investigators also discovered two White House advance team members checked in prostitutes as overnight guests at a Cartagena hotel in the days before President Obama's April 13 visit. Three U.S. delegation members that stayed at the Hilton brought prostitutes back as overnight guests. One of them was ours (Secret Service) and the other two were White House staffers," a high-ranking Secret Service official told FoxNews.com. We knew very early that White House staffers were involved. Twelve of the 13 agents investigated for alleged misconduct in Cartagena stayed at another hotel, the El Caribe. Only one of those charged with misconduct had a room at the Hilton, where President Obama and the White House advance team also stayed. White House Press Secretary Jay Carney said in April, just days after Obama's visit, that there was no indication any White House advance team members were involved in the prostitution scandal. But whether there will be any reference to the White House staffers in the upcoming report, from the Department of Homeland Security's Office of Inspector General, remains to be seen. The report has yet to be delivered, though DHS OIG officials said Tuesday it would be submitted in the coming days. Acting DHS Inspector General Charles K. Edwards initially told a congressional panel in May he was aiming to present it by July 2. The delay has sparked speculation the report was being altered or manipulated to conceal or minimize the roles of some of those involved, multiple Secret Service officials with senior leadership positions told FoxNews.com. Meanwhile, Senators Susan Collins of Maine and Joseph Lieberman of Connecticut, of the Senate Homeland Security Committee sent Edwards a letter on Sept. 14 asking for information about the status of the report. A congressional source told FoxNews.com the Senate committee staff is particularly eager to see the report because it "includes information that two members of the White House advance team had prostitutes overnight." "The Committee wants to know if White House staff engaged in improper conduct in Cartagena, which the White House previously denied," the source added. "We are writing to inquire about the status of the investigation we requested into the April 2012 incidents in Cartagena, Colombia, involving the U.S. Secret Service and possibly other federal personnel and certain foreign nationals," the letter said. It was not clear what level of White House advance team personnel were involved or if they had access to classified material about the president's visit. The letter contained questions including the date the draft report was completed; if the report had been submitted to Secret Service and DHS for comment; and whether those comments will be identified separately in the final report. It also asks: "Have any changes been made to the report in response to comments received from the Secret Service or the Department?" "Obviously we're worried the draft version of the report -- what the DHS IG investigators found on the ground in Cartagena -- is going to get changed and edited before the final version gets out," said a Secret Service source with knowledge of the IG's initial findings. Collins and Lieberman and the committee sent this letter in part to make sure that nobody will water down the final report. The committee sending the letter will prevent them from tampering with the report, that's one of the reasons why they sent it. In response to multiple requests for comment, a spokesman with the DHS OIG on Tuesday afternoon denied the report was being deliberately delayed. "We have completed our independent review, which was requested by Congress, and are currently in the final stages of preparing the final report of investigation," spokesman Bill Hillburg said. He said DHS Secretary Janet Napolitano would be briefed in the "next few days," and Collins and other lawmakers after that. Press Secretary Carney's statement, on April 23, came well before the DHS investigation was complete, but ruled out any wrongdoing by White House advance staff. "There have been no specific, credible allegations of misconduct by anyone on the White House advance team or the White House staff," Carney said at a press briefing. Nevertheless, out of due diligence, the White House Counsel's office has conducted a review of the White House advance team, and in concluding that review, came to the conclusion that there's no indication that any member of the White House advance team engaged in any improper conduct or behavior. The White House did not return FoxNews.com's request for comment on Tuesday afternoon. DHS spokesman Matt Chandler said on Tuesday that Napolitano's office had not yet received the report. He added: "Furthermore, the OIG is independent and solely and separately determines the timing of its reports. I would refer you to the OIG for further questions. Edwards said during congressional testimony on May 23 that he hoped to have the initial report on the Cartagena incident completed by July 2. "We plan to interview Special Agent in Charge Paula Reid, who had on-site responsibility for the Secret Service's Cartagena detail," Edwards said. We also plan to interview (Secret Service) Director (Mark) Sullivan. We will review the Secret Service's report on its internal investigation as soon as it becomes available. Contingent upon our receipt of that report, our goal is to complete the first phase of our review and report our findings by July 2nd. Multiple high-level officials, including current and former Secret Service agents, have told FoxNews.com that they believe Sullivan has essentially covered up White House involvement in the scandal, while publicly skewering agency employees who were involved. Secret Service spokesman Ed Donovan said his agency "has been completely cooperative with inquiries from DHS OIG on all matters related to the investigation of events that transpired in Cartagena." Thirteen Secret Service agents, including one female agent, were investigated for their alleged involvement with prostitutes at Cartagena hotels on the night of April 11 and early morning of April 12, in advance of Obama's arrival on April 13 for a Summit of the Americas meeting. A dozen members of the military were also investigated for alleged misconduct during the same visit, as were two DEA agents who were alleged to have had conduct with prostitutes at an apartment maintained by the agency in Cartagena. All but a few of the 13 Secret Service agents have retired, resigned, or are on administrative leave and have had their clearances suspended pending revocation hearings or appeals. Last week, two of the agents were notified by their attorneys that their security clearances had been officially revoked. One of two supervisors had his clearance revoked and is appealing the revocation; the other supervisor retired. A number of other agents initially sent home from Colombia and investigated were brought back on the job after saying during polygraph examinations that they did not know the women in their rooms were prostitutes. Those back on the job have been reassigned to other divisions of the Secret Service. Contact Jana Winter at jana.winter@foxnews.com FoxNews.com's Judson Berger contributed to this report.