WASHINGTON (AP) — A Secret Service scandal involving prostitutes in Colombia that has overshadowed President Barack Obama's diplomatic mission in Latin American probably isn't an isolated incident, and the agency should ensure it doesn't happen again, a leading House Republican said Sunday

California Rep. Darrell Issa, chairman of a House investigative panel, said he wasn't certain whether Congress would hold hearings on the misconduct. But lawmakers will be looking "over the shoulder" of the Secret Service, he said, to make sure that the agency's method for training and screening agents isn't endangering the nation's VIPs.

"Things like this don't happen once if they didn't happen before," said Issa, who leads the House Oversight and Government Reform Committee.

Eleven Secret Service employees are on administrative leave for misconduct and five service members assigned to work with the agency are confined to quarters amid allegations that a group of personnel partied with prostitutes before Obama arrived in Colombia for the weekend summit with Latin American leaders.

Obama, who hasn't addressed the scandal publicly, could make his first comments about it during a late afternoon news conference Sunday in Cartagena with Colombia's president.

White House spokesman Jay Carney has dismissed suggestions that the incident had distracted the president.

"I think it's been much more of a distraction for the press," Carney said Saturday. "He's here engaging in the business that he came here to do with the assembled leaders of the Americas."

Issa told CBS' "Face the Nation" that lawmakers are trying to confirm the precise number of U.S. personnel involved in the scandal. He said the number could be higher than initially thought.

Of primary concern, he said, was that agents who behave badly are vulnerable to blackmail and therefore risk the security of the president and others under their protection.

"We have got to ask, 'where are the systems in place to prevent this in the future?" Issa said. "It's the reason that the investigation will not be about the 11 to 20 or more involved. It will be about how did this happen and how often has this happened before?"

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He later added: "It's not about whether the president was in danger this time. It's whether or not you need to make changes so the American people can have confidence in all of their workforce."

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