Story highlights Evidence "indicates that this likely was not just a one-time incident," senator says

The scandal first focused on Secret Service agents and military personnel

They were in Colombia in advance of Obama's trip to a summit there

Three Drug Enforcement Administration agents are under investigation for allegedly hiring prostitutes in Cartagena, Colombia, a congressional source confirms to CNN.

According to this source, House Homeland Security Chairman Peter King, R-New York, and committee investigators have been "aware of this for some time."

News of the investigation comes on the heels of a prostitution scandal involving U.S. military and Secret Service agents who were detailed to Colombia in April in advance of President Barack Obama's trip to the Summit of the Americas. Several Secret Service members have been dismissed as a result of investigations.

JUST WATCHED Prostitution easily found in Cartagena Replay More Videos ... MUST WATCH Prostitution easily found in Cartagena 03:39

JUST WATCHED Prostitutes not stealing U.S. secrets Replay More Videos ... MUST WATCH Prostitutes not stealing U.S. secrets 02:15

JUST WATCHED Brazilian to sue in prostitution scandal Replay More Videos ... MUST WATCH Brazilian to sue in prostitution scandal 03:15

JUST WATCHED Escort calls Secret Service 'dumb men' Replay More Videos ... MUST WATCH Escort calls Secret Service 'dumb men' 01:55

CNN spoke with three senators -- one of whom asked not to be identified -- who confirmed the investigation concerning the DEA agents. Sen. Patrick Leahy of Vermont, the Judiciary Committee chairman, said senators found out about the matter a week ago but were asked to keep it quiet until the agents were removed from Colombia.

"Not good," said Sen. Susan Collins of Maine, the top Republican on the Senate Homeland Security Committee, which on Wednesday will hear the first testimony from Secret Service Director Mark Sullivan.

Collins released a statement later Monday, saying, "It's disturbing that we may be uncovering a troubling culture that spans more than one law enforcement agency.

"In addition to the Secret Service scandal, we now learn that at least two DEA agents apparently entertained female foreign national masseuses in the Cartagena apartment of one of the agents. The evidence uncovered thus far indicates that this likely was not just a one-time incident," Collins added in her statement.

DEA spokesman Rusty Payne said the matter has been turned over to the Justice Department inspector general.

"The Drug Enforcement Administration was provided information from the Secret Service unrelated to the Cartagena hotel Secret Service incident, which DEA immediately followed up on, making DEA employees available to be interviewed by the Department of Justice's Office of Inspector General. DEA takes allegations of misconduct very seriously and will take appropriate personnel action, if warranted, upon the conclusion of the OIG investigation," Payne said.

In addition, Collins and one other senator told CNN that one additional Secret Service agent has come forward in recent days and volunteered to his superiors that he paid a prostitute while in Colombia in advance of President Obama's recent trip there. An aide to Collins said the Secret Service is telling the senator's office that the agent says he thought he was paying for a massage, not for prostitution.

Because that agent came forward on his own, he will not lose his job, one of the senators said.