* Senator questions whether security could have been compromised
* Secret Service agents, Marines under investigation
* Women being interviewed in Colombia (Adds comments from congressman Peter King)
By Susan Cornwell and Tabassum Zakaria
WASHINGTON, April 17 (Reuters) - U.S. Secret Service agents and military personnel took as many as 21 women back to their hotel in Colombia in an incident last week involving alleged misconduct with prostitutes, a Republican senator said on Tuesday.
"There are 11 agents involved. Twenty or 21 women foreign nationals were brought to the hotel, but allegedly Marines were involved with the rest," Senator Susan Collins of Maine - who was briefed by the director of the Secret Service, Mark Sullivan - said in an email to Reuters.
"Director Sullivan is rightly appalled by the agents actions and is pursuing a vigorous internal investigation," said Collins, the senior Republican on the Senate Homeland Security Committee. "He ordered all the agents to return to Washington immediately, and all have been interviewed."
More details about the incident that marred President Barack Obama's weekend trip to the Summit of the Americas in Categena, Colombia emerged on Tuesday.
A Marine Corps spokesman said two Marine dog handlers assigned to support the Secret Service are being investigated to determine whether they were involved in soliciting prostitutes.
The spokesman, Captain Kevin Schultz, said the Marines were on the advance team and were not in direct contact with Obama.
The Secret Service has revoked the top security clearances of its 11 agents and placed them on administrative leave due to the incident.
The agents brought the prostitutes to their beachfront hotel in Cartagena, before Obama arrived for the summit, according to a local police source. A U.S. official told Reuters that more than 10 military service members also may have been involved.
"The president has confidence in Director Sullivan," Obama's spokesman Jay Carney said. "The director acted swiftly in response to this incident and is overseeing an investigation."
INVESTIGATORS IN COLOMBIA
Another lawmaker who has spoken with Sullivan, Representative Peter King, said that investigators in Colombia for the Secret Service were interviewing the women involved, who had left identification at the hotel desk, King told Reuters.
"So far there is no information that any of them were involved with any narco-terrorist group or any organized crime. Nor is there any indication that the women were looking for the men," he said, addressing a concern that the agents might have been set up by someone with intent to compromise them.
King said he understood that each of the 11 agents took a woman to his room.
"A number (of the agents) are saying they did not consider them prostitutes," he said.
Addressing another security concern, King said he had been told there were no schedules of Obama's trip to Colombia in the agent's rooms.
King, a Republican who chairs the House of Representatives Homeland Security Committee, said he had not decided whether to hold hearings on the incident, noting that he had not seen such behavior before from Secret Service agents.
Collins said she asked Sullivan several questions, including who the women were. "Could they have been members of groups hostile to the United States? Could they have planted bugs, disabled weapons, or ... jeopardized security of the president or our country?"
She also asked whether there was evidence of previous misconduct, and "given the number of agents involved, does this indicate a problem with the culture of the Secret Service?"
Sullivan promised to keep her updated and Collins said she was confident he would fully investigate and "pursue appropriate action against the agents should the allegations prove true."
NBC News reported that two Secret Service supervisors and three counter-assault team members were among the 11 employees implicated in the alleged misconduct in Colombia.
A Secret Service source told Reuters the employees involved were support personnel and not part of the advance team, so they would not have had detailed itineraries and schedules that far ahead of the president's arrival.
Also any such papers would not be left in the hotel room, but would be kept in a security room set up for foreign presidential trips and guarded by Marines.
"That kind of stuff is not kept in rooms," the Secret Service source said.
A report from ABC News said Secret Service agents partying at a club in Cartagena boasted that they worked for Obama and were in Colombia to protect him. (Additional Reporting By Emily Stephenson. Editing by Jackie Frank and Christopher Wilson)