Washington (CNN) -- The ranking Republican on the Senate Judiciary Committee has asked the U.S. Secret Service about the potential involvement of White House staff in the prostitution scandal in Colombia.
Sen. Chuck Grassley questioned Secret Service Director Mark Sullivan and Acting Inspector General Charles Edwards in letter Friday about whether they are investigating the possibility that the White House Communications Agency and the White House Office of Advance may be involved in the controversy.
He asked specifically if the agency reserved rooms or shared them with White House staff members for operational matters.
White House Press Secretary Jay Carney, speaking at a daily briefing Friday, said he has no reason not to believe that the scandal involved "anything but the agents and the military personnel," criticizing those seeking to politicize the issue.
"It is preposterous to politicize the Secret Service," said Carney, responding to a question regarding criticism of the Obama administration by former Alaska Gov. Sarah Palin and other Republicans. "What they're doing is trying to turn these incidents, one that's still under investigation, into political advantage."
White House staff have not been implicated in the controversy, which emerged last week.
On Friday, the Secret Service announced that three more of its employees have "chosen to resign" in the wake of the scandal.
Six Secret Service members now have left their jobs in the wake of the incident in Cartagena, Colombia, which came while they were on a security detail in advance of President Barack Obama's trip there for the Summit of the Americas.
The agency also announced Friday that a 12th Secret Service "employee has been implicated," having previously said 11 were under investigation.
One employee "has been cleared of serious misconduct, but will face administrative action," the Secret Service said.
Five employees are on administrative leave and have had their security clearances temporarily revoked.
In addition, the U.S. military is investigating 11 of its own troops for possible heavy drinking and consorting with prostitutes.
Secret Service Director Mark Sullivan -- who briefed Obama on the investigation Friday, according to White House officials -- has ordered a "comprehensive" investigation of everything that happened during the trip.
That includes interviews with every Secret Service member on site, hotel staff and alleged prostitutes, the source said. In addition, the source confirmed that Secret Service agents were staying at a second hotel on the trip -- identified as a Hilton in Cartagena -- which presumably will be included in the expanded probe.
The controversy has embarrassed the nearly 150-year-old agency that protects the president and other top officials and investigates criminal activity. It also raised questions about a possible security breach immediately preceding Obama's visit, though House Homeland Security Committee Chairman Rep. Peter King has said that "from everything we know, nothing was compromised."
"The concern I have is about illegal leaks coming from apparently rogue elements within the Secret Service of privacy-protected information," said attorney Lawrence Berger, general counsel for the Federal Law Enforcement Officers Association which represents Secret Service agents and other said. "It is distorting the review of what happened."
All the employees are accused of cavorting prostitutes ahead of last week's visit by Obama. They'd arrived earlier that morning as a part of the "jump team" that flies in on military transport planes with vehicles in the president's motorcade.
According to sources, the alleged prostitutes -- the youngest of whom were in their early 20s -- signed in at the Cartagena hotel, where Secret Service members apparently stayed, flashing their local ID cards.
One of these women allegedly was later involved in a dispute about how much she was to be paid for the night, which brought the entire incident to light and sparked controversy in the United States and Colombia.
Former Colombian President Alvaro Uribe told CNN on Thursday that the incident was due entirely to "a lack of ethics (on the part of) the Secret Service of the United States."
Members of the U.S. Congress offered similarly biting remarks. House Minority Leader Nancy Pelosi called the allegations "disgusting," while Senate Majority Leader Harry Reid described the agents' alleged actions as "either really stupid or a total lack of common sense."
The U.S. military is investigating six members of its elite Army Special Forces, or Green Berets, officials said.
While soliciting prostitution is in most cases legal for adults in Colombia, military law bars service members from patronizing prostitutes, engaging in conduct unbecoming an officer or, for enlisted personnel, conduct "prejudicial to good order and discipline." It is also considered a breach of the Secret Service's conduct code, government sources said.
CNN's Carol Cratty, Briana Keilar, Brian Todd, Bob Kovach, John King, Barbara Starr, Deirdre Walsh, Rafael Romo, Jessica Yellin and Tom Cohen contributed to this report.