© Manuel Pedraza / AFP - Getty Images fileView of the Hotel Caribe in Cartagena, Colombia, where a prostitution scandal involving U.S. Secret Service agents erupted in April 2012.
Two U.S. Drug Enforcement Administration agents "facilitated a sexual encounter" between a prostitute and a U.S. Secret Service agent days before President Barack Obama visited Colombia for a summit meeting in April 2012, according to a Justice Department investigation obtained exclusively by NBC News.

A summary of the findings of the investigation, included in a Dec. 20 letter from the Justice Department's Office of the Inspector General to Sens. Joseph Lieberman and Susan Collins, indicated that a third DEA agent present on the night of the incident was not involved in procuring the prostitute for the Secret Service agent.

"While DEA agent #3 was present for a dinner that took place earlier that evening with the USSS agent and the other two DEA agents, he was not present in the residence when the sexual encounter took place and played no role in facilitating it," the summary said.

All three DEA special agents admitted that they had paid for sexual services of a prostitute, the investigation also found, and "used their DEA Blackberry devices to arrange such activities." In addition, the report says the agents tried to destroy incriminating information or initially lied to investigators about the incidents. All three agents have high-security clearances.

The summary concluded that the agents' actions did not warrant criminal prosecution. It said the U.S. Attorney's Office also "declined to initiate legal proceedings." It said the case had been referred to the DEA for "action it determines to be appropriate."

In a letter sent Wednesday to DEA Administrator Michele Leonhart, Collins, R-Maine, called the findings "troubling" and the conclusion that law enforcement officials obstructed the OIG investigation "deeply troubling." She also asked Leonhart to explain why - nearly four months after the Office of the Inspector General referred the matter to the DEA -- "It is my understanding that those administrative actions are still pending."

DEA spokesman Rusty Payne said the agency would not comment at this time. "The DEA has just received the letter and will respond to the senator at the appropriate time," he said.

A DEA official, speaking on condition of anonymity, said that while senior officials learned of the OIG's findings in September, the agency is not allowed to act until the OIG concluded and presented its findings on Dec. 20. The official also pointed out that DEA agents, like most other federal employees, are members of the civil service and therefore can't be fired or stripped of their security clearance without a multi-layer process.

Sources briefed on the scandal have told NBC News that approximately a dozen members of the Secret Service security detail hit the clubs of Cartagena on the evening of April 11, 2012, for a night of drinking that ended with them bringing women back to their hotel rooms.

Some of the women received money, and others did not ask for any, but in one case, an agent refused to pay, and the woman summoned a police officer. That led to an angry encounter that resulted in the debauchery becoming public.

The scandal led to the resignation or retirement of nine of 12 implicated Secret Service employees. Three others were cleared of serious misconduct but still could be disciplined.

The Pentagon also launched an investigation of a dozen military members in connection with the scandal. No disciplinary action has been announced as a result of that investigation.