US Homeland Security chief Janet Napolitano assured probing senators on Wednesday that the Secret Service agent misconduct in Colombia never jeopardised President Barack Obama's safety.
Napolitano pledged a thorough investigation into the prostitution row that overshadowed the Summit of the Americas in the Caribbean resort of Cartagena, but said she and Obama had "full confidence" in the agency's director.
Senior Republican Senator John McCain fumed that he was being "denied access" to key information after military officials briefed him on a Pentagon probe about the scandal.
"We will leave no stone unturned," Napolitano told a Senate hearing on the activities of the Homeland Secretary Department, which oversees the Secret Service.
The agent's director Mark Sullivan "has the president's and my full confidence as this investigation proceeds," Napolitano said.
A panel of senators grilled her on the misconduct by agents alleged to have brought prostitutes back to their hotel in Cartagena, Colombia while preparing for Obama's attendance at the regional summit.
Asked directly whether the safety or security of the president was ever questioned, she said she had immediately quizzed Sullivan about that.
"The answer is no. There was no risk," she said.
Eight Secret Service agents have been dismissed so far.
"The Secret Service is moving to permanently revoke the security clearance of another, and three of the employees involved have been cleared of serious misconduct," Napolitano said.
But she stressed that "the allegations are inexcusable, and we take them very seriously".
Twelve members of the military, including a member of the White House Communications Agency, are also being probed by the Pentagon.
The US agents and personnel had gone to Cartagena to prepare security for Obama's arrival for the Summit of the Americas and the affair morphed into a huge diplomatic embarrassment for the United States.
Obama has described the agents involved as "knuckle heads," but said their transgressions should not detract from the wider work of the Secret Service.
Democratic Senator Patrick Leahy, who chairs the Judiciary Committee that oversees homeland security, said utmost rigour was needed to ensure the protection of top American officials, including the president and his Republican challenger Mitt Romney.
"No one wants to see the president's security compromised or America embarrassed," Leahy said.
"I can't think of anything, aside from a personal tragedy, that would look worse to the rest of the world if something happened either to President Obama or governor Romney, especially during a presidential election."
With nothing short of the president's security at stake, McCain, the top Republican on the Armed Service Committee, sought a briefing from senior military staff.
"Unfortunately, nearly two weeks after the events in Colombia, the briefers sent by the Department of Defence were woefully unprepared to answer even the most basic questions about what happened in Cartagena, and provided appallingly little new information," McCain said in a statement.
"We need to know the facts. We need to know the impact of this potential misconduct, which occurred less than a day, or perhaps hours, before the president arrived in Cartagena, on the performance of the military Joint Task Force charged with his security
"Yet, we are being denied access to the information we need in order to make informed judgments or take needed actions. This is entirely unacceptable."
Napolitano said she saw no evidence of a problem with the agency as a whole, insisting there have been no complaints of similar misbehavior at any time during the past two-and-a-half years, a period in which the Secret Service provided protection on 900 foreign trips and 13 000 domestic trips.
Republican Lindsey Graham suggested the problem stretched beyond Cartagena.
"Obviously there was a system failure here," the South Carolina senator said, charging that Napolitano faced an "order and discipline problem" within the agency.