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The lead federal investigator into the Colombia prostitution scandal said for the first time Friday that White House personnel may have been involved -- despite administration claims to the contrary.

Charles Edwards, the acting inspector general for the Department of Homeland Security, wrote in a letter to Sen. Susan Collins that his office's investigation into the April incident found "a hotel registry that suggests that two (non-Secret Service) personnel may have had contact with foreign nationals."

The letter came two days after a FoxNews.com report revealed possible White House advance team involvement.

One of those employees, Edwards wrote, was a Defense Department employee "affiliated" with the White House Communication Agency.

The other, he said, "may have been" affiliated with the White House advance team.

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The White House denied the claims Friday. And Edwards wrote that his office did not pursue those leads "because they are not DHS personnel."

Edwards wrote that while the allegations that went beyond the Secret Service "were outside the scope of the investigation, one of these employees is a Department of Defense employee affiliated with the White House Communication Agency and the other, whose employment status was not verified, may have been affiliated with the White House advance operation."

That statement, though, calls into question claims made back in April by White House Press Secretary Jay Carney about the possible involvement of the White House team. Carney said that the White House counsel's office conducted a review and "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 Obama administration stood by its original claims in reaction to Edwards' statements.

A senior administration official said the member of the advance team was a "volunteer," as opposed to a White House employee. Further, the official said the volunteer was wrongly implicated based on inaccurate hotel records.

White House spokesman Eric Schultz backed up Carney in standing by the original White House review.

"As we've said for months, the White House review concluded that no members of the White House advance team, either staff or volunteers, engaged in inappropriate conduct during the president's trip to Colombia," Schultz said.

Collins said in a statement Friday she was "troubled" by the new revelations, though, citing Carney's original claim.

"On April 23, the White House spokesman said that the White House counsel's office had conducted its own review and concluded there had been no credible allegations of misconduct by anyone on the White House advance team or the White House staff. The White House explicitly denied any involvement after its own investigation and now the IG is questioning that account. This raises concerns about the credibility of the White House investigation," she said.

A Secret Service source who was on the April Cartagena trip also questioned Edwards' claim that the leads were not pursued. The source told FoxNews.com that agents on the trip were specifically asked about White House staff, and that those leads were pursued by the DHS investigators.

"He is lying that (White House) leads weren't pursued. I was asked by ... investigators if I knew about (White House) staff with girls along with many other (Secret Service) types that were interviewed," the source said.

"He is playing with the truth," the source said of the IG's claim that White House leads weren't followed. "We were questioned specifically about the White House staff. We were all questioned."

Edwards' letter came in response to an inquiry from Collins about the status of the DHS internal investigation. A spokesman for the DHS inspector general told FoxNews.com earlier this week that the investigation had recently been completed and the report would be submitted to administration officials in a matter of days.

On Wednesday, FoxNews.com reported exclusively that the probe conducted by Department of Homeland Security investigators into the Cartagena scandal turned up claims that two members of the White House team had checked in prostitutes to the Hilton hotel in Cartagena.

When Carney was asked Thursday about the forthcoming investigative report on the Colombia prostitution scandal, and the FoxNews.com report, Carney noted that the report was not yet public and that the White House had not seen it.

"I have no comment on an IG report that nobody has seen," he said. "We'll await the report."

Carney, though, said the White House stands by its counsel's office report from April concluding no White House staff involvement.

Edwards, in his letter, asked to schedule a time with Collins to provide "a personal briefing on the scope of the investigation and a summary of our general findings."

The letter to Collins was also sent to Sen. Chuck Grassley, R-Iowa, who fired off a letter of his own on Thursday to White House Counsel Kathryn Ruemmler demanding answers about the alleged White House personnel involvement.

Grassley, R-Iowa, ranking member of the Senate Judiciary Committee, cited the earlier FoxNews.com report and asked a string of pointed questions -- namely, he noted the White House never got back to him with more information about its counsel review and asked about the forthcoming investigative report.

"Now more allegations have arisen regarding White House personnel procuring prostitutes during the President's trip to Colombia," Grassley wrote.

He asked whether the White House has been in contact with the inspector general's office, whether the White House had any input in drafting the report and if the Secret Service was aware of alleged involvement of White House personnel. "If the report includes the findings that are being reported how did your initial review fail to uncover the involvement of these staffers?" Grassley wrote.

The letter from Edwards also provided additional details about the original investigation into the Secret Service.

Edwards wrote that investigators interviewed 251 Secret Service personnel, as well as reviewed travel records and hotel registries.

"Based on these interviews and the review of these records, we identified thirteen individuals who were then employed by the USSS (Secret Service) who had personal encounters with female Colombian nationals consistent with the misconduct reported around the time of advance activities for the President's visit to Cartagena," he wrote.

He said the encounters took place at the Hotel Caribe, the Hilton Cartagena and at a private residence. "Our investigation determined that twelve of these thirteen USSS employees met the female Colombia nationals at bars or clubs and returned with them to their rooms at the Hotel Caribe or the Hilton Cartagena Hotel. In addition, one USSS employee met a female Colombian national at private residence," he wrote.

Three of the women left without asking money, five asked for money and were paid and four asked for money and were not paid, Edwards wrote. Police arrived on the scene when one of those women complained.

"Although we found that these agents engaged in misconduct, our investigation developed no evidence to suggest that the actions of USSS personnel in Cartagena compromised the safety and security of the President or any sensitive information during this trip," he wrote.

A voicemail left for a spokesman for the DHS inspector general requesting comment on the letter was not immediately returned.

Contact Jana Winter at jana.winter@foxnews.com

FoxNews.com's Judson Berger contributed to this report.