More Secret Service agents involved in scandal?

WILLIAMS: Good evening.

BRIAN WILLIAMS, anchor: Mention the US Secret Service to most Americans and it will conjure up an image of the brave and silent protectors of the president, the men and women wearing suits and earpieces and sunglasses, who have sworn to take a bullet for the person they're protecting if it comes to that. But after this weekend most mentions of the US Secret Service have to do with an unfolding scandal. The members of a joint Secret Service military advance party , who were in Cartagena , Colombia , prior to the president's trip there this weekend. They were sent home after allegations of an altercation involving prostitution . And as we'll report here tonight, it's getting worse and it's widening, and it's triggering questions about the very culture of a special organization. We begin tonight with our White House correspondent Kristen Welker , who accompanied the president to Colombia and back. Kristen , good evening.

KRISTEN WELKER reporting: Good evening, Brian . Well, investigators at the Secret Service , the Pentagon and on Capitol Hill are trying to figure out exactly how this could have happened. And tonight, we are learning that all 11 Secret Service personnel have had their security clearances pulled. According to US officials, at least 11 Secret Service and more than five military personnel are involved in the widening prostitution scandal . NBC 's national investigative correspondent Mike Isikoff has been working this story.

MICHAEL ISIKOFF reporting: Sources tell us among those involved were two Secret Service supervisors, three members of the elite counter-assault team, whose job it is to repel attacks on the president's motorcade, and three members of the counter- sniper team , those officers with rifles who scout for threats from the tops of buildings.

WELKER: They were all a part of an advance team, preparing security for the president's diplomatic trip to Colombia . Isikoff says investigators are looking into the possibility that more may be involved.

ISIKOFF: We're told all of those involved had hard copies of the president's day-by-day, minute-by-minute schedule. And if the prostitutes had access to that, they could have potentially given it to a foreign intelligence service , a drug cartel or even a terrorist group.

WELKER: It happened last Wednesday night when the advance team allegedly brought prostitutes back to their hotel, just a few minutes from where the president would eventually stay. The hotel learned about the incident the next morning after one of the women fought with an agent over money. The hotel alerted local authorities and eventually it went all the way to the White House .

President BARACK OBAMA: We're representing the people of the United States . And when we travel to another country I expect us to observe the highest standards.

WELKER: Sources tell NBC News the military officials under investigation include bomb disposal experts and dog handlers from the Army , Navy and Marines .

General MARTIN DEMPSEY (Joint Chiefs of Staff Chairman): We're embarrassed by what occurred in Colombia . We let the boss down.

WELKER: In a statement the Secret Service said, "These actions have had no impact on the Secret Service 's ability to execute a comprehensive security plan for the president's visit to Cartagena ." Representative PETER KING (Republican, House Homeland Security Committee Chair): And this certainly could have compromised the president's security, it could have compromised national security .

WELKER: The scandal is the latest incident that has fueled the agency's most vocal critics' questions about its effectiveness.

Mr. RON KESSLER (Author of "In the President's Secret Service"): There's a culture in the Secret Service that's fostered by the management of just nodding, winking, favoritism.

WELKER: Now, prostitution is legal in parts of Colombia but all of the agencies involved say the alleged misconduct is a clear violation of their

standards. Brian: Kristen Welker starting us off at the White House . Kristen ,

WILLIAMS: