(CNN) -- One of the three Drug Enforcement Administration agents under investigation for allegedly soliciting sex in Cartagena, Colombia, had a long-term relationship with a prostitute, potentially exposing himself to blackmail by drug cartels or other criminals, according to two government sources familiar with the investigation.
Investigators at the Department of Justice and the DEA are trying to determine if that woman was aware that the man she had multiple liaisons with was stationed by the DEA in her drug-torn country. Officials have tried to interview her -- even compelling the agent to get her on the phone -- but have not been able to talk to her yet, the sources said.
"The reason this is troubling is that these women are really unknown to our law enforcement personnel," said Sen. Susan Collins of Maine, the top Republican on the Senate Homeland Security and Governmental Affairs Committee, which is investigating the use of prostitutes by Secret Service agents in Colombia before President Barack Obama's visit there last month.
"We don't know if they are associated with drug cartels. Whether they have been sent into entrap the agents or compromise them in some way which could endanger their mission -- whether it's protecting the president in the case of the Secret Service agents, or whether it's pursuing drug activities in the case of the DEA agents."
The DEA agents were not involved in the security for the president's trip, the government sources said.
The DEA agent's relationship with the prostitute came to light after a Secret Service agent voluntarily reported to his superiors that he was at a party at the agents' Cartagena apartment on April 13 with the three DEA agents and several women, according to the government sources. One of the sources said the Secret Service agent reported that he went into a bedroom with one of the women and had "a massage with a sexual aspect to it."
Later, the Secret Service agent witnessed the women he was with being paid in local currency by one of the DEA agents, the sources said. But the Secret Service agent has insisted to investigators that it was not his intention to pay for sex.
"Because of all the pressure, he wanted to self-report and basically confess that this had occurred instead of getting caught," said a government source who described the Secret Service agent as "guilt-ridden."
The Secret Service agent is the 13th agency employee to get caught up in the Colombia prostitution scandal. He is currently on administrative leave but is not expected to lose his job because he came forward on his own to report the incident, according to several sources.
House Homeland Security Committee Chairman Peter King, R-New York, told CNN the Secret Service agent took and passed a polygraph test. But he said the alleged behavior by the DEA agents puts lives at risk.
King said any time U.S government personnel are "involved in any sexual manner...you are leaving yourself, at the very least, you're opening yourself to blackmail. Secondly in these countries like Colombia or the old Soviet bloc, you run the risk (of people) being recruited to get something on you or to drug you to get information from you, to compromise you, to kidnap you."
The Colombia scandal will be the subject of a hearing Wednesday before Sen. Collins' committee. Secret Service Director Mark Sullivan will testify at the hearing in his first public appearance before Congress about the incident.
In remarks prepared for delivery as her opening statement at the hearing, Collins says, "This reckless behavior could easily have compromised individuals charged with the security of the president of the United States."
She adds, "The facts so far lead me to conclude that, while not at all representative of the majority of Secret Service personnel, this misconduct was almost certainly not an isolated incident."
CNN Congressional Correspondent Dana Bash contributed to this report.