Investigators have determined that as many as 20 U.S. Secret Service and military personnel might have been involved in the hotel misconduct in Cartagena, Colombia, as the scandal that erupted during President Obama’s trip to the country last week put high-level officials on the defensive.

A preliminary investigation by the Defense Department, which included a review of video from hotel security cameras, found that nine military personnel were possibly involved in the carousing at the center of the probe, congressional sources familiar with the probe said. Already, 11 Secret Service agents have been placed on leave amid allegations they entertained prostitutes, potentially one of the most serious lapses at the organization in years.

Two of the Secret Service personnel are paid at one of the higher levels of the federal pay scale, meaning they are senior officials potentially in supervisory positions, according to a congressional official with knowledge of the investigation.

The accusations are triggering scrutiny of the culture of the Secret Service — where married agents have been heard to joke during aircraft takeoff that their motto is “wheels up, rings off” — and raising new questions at both the agency and the Pentagon about institutional oversight at the highest levels of the president’s security apparatus.

“We are embarrassed,” Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, told reporters in a briefing at the Pentagon. “We let the boss down, because nobody is talking about what went down in Colombia other than this incident.”

At the same time, details emerged about the night of partying Wednesday that led to the scandal. People in Cartagena familiar with the matter said that some of the Secret Service agents paid $60 apiece to owners of the Pleyclub, a strip club in an industrial section of Cartagena, to bring at least two of the women back to the Hotel Caribe, where Obama’s advance team was staying.

The following morning, one of the women demanded an additional payment of $170, setting off a dispute with an agent that drew the attention of the hotel, the Cartagena sources said.

According to the Pleyclub’s registry at the local chamber of commerce, one of the club’s owners is named Michael Adam Hardy, whom chamber officials described as either American or Canadian.

On Monday, the Secret Service moved to revoke the top-secret security clearances of all 11 men from the agency who are under investigation, spokesman Edwin Donovan said.

The revocation of such clearances is not uncommon, he emphasized, and security clearances can be reinstated after internal investigations are complete, depending on the findings.

In a letter to all agency employees, Secret Service Director Mark Sullivan stressed that it is “imperative . . . to always act both personally and professionally in a manner that recognizes the seriousness and consequence of our mission.”

Sullivan promised a “thorough and fair” investigation and concluded by saying that “in the wake of this embarrassing incident, it is my hope that each of us will be steadfast in our efforts to ensure that our performance and behavior mirror the oath we have sworn to uphold.”

The Secret Service personnel under suspicion include a mix of special agents who provide personal protection for the president and uniformed officers who perform building security and logistical support. They were part of a U.S. advance team of up to 200 people sent ahead to prepare for Obama’s arrival.

After the allegations of misconduct came to light Thursday, when hotel staff notified the U.S. Embassy, the service removed the 11 agents and replaced them with a new team. In addition, the military confined five of its personnel to their rooms at the hotel, pending the investigation. Military officials did not say how many more of its personnel might now be suspected of participating in the alleged misconduct.

Prostitution, legal and regulated, is a booming business in the Caribbean tourist hub of Cartagena, a city of about 1 million inhabitants that is famous for its Spanish colonial heart and a modern stretch of Miami-style high-rises. As a byproduct of its lure of cruise ships and conventioneers, Cartagena draws prostitutes from both the city’s poor and upper-class echelons — as well as from different cities around the country.

Before the summit, the government of President Juan Manuel Santos asked the city health department for an action plan outlining disease prevention efforts with prostitutes ahead of the gathering of 30 hemispheric leaders.

Officials at Cartagena’s health department said that there are about 80 streetwalkers in the city’s colonial district, which features bountiful nightclubs, boutique hotels and elegant restaurants. Another 550 women, who will spend the night with a client for about $250, are estimated to be available at 15 nightclubs, officials said.

The Pleyclub touts its services by distributing small, glossy advertisements featuring nearly naked women to taxi drivers who drive visitors around town. The ads, in Spanish, promise: “We’re the best good time in the city.”

Inside the Pleyclub, there is a stage with two poles and a glass-enclosed shower in which women perform strip shows.

Several hotel workers said some of the Secret Service agents spoke good Spanish.

The Hotel Caribe, like most hotels in Cartagena, permits overnight visitors to join hotel guests. But there are rules: Young women brought for the night must come after 11 p.m.; cannot spend time in public areas, such as the lobby; must present identification to prove they are adults; and must leave by 6 a.m., two hotel employees said. The hotel also levies a $60 surcharge for each overnight visitor, said the employees, who spoke on the condition of anonymity because they were not authorized to speak for the management.

Hotel management declined to comment about the incident, saying that it must protect the privacy of its guests.

Even though Secret Service officials have said Obama’s security was not compromised, lawmakers who oversee the agency have grown increasingly outraged as new allegations surface.

“I find this to be so appalling,” said Sen. Susan Collins (R-Maine), the ranking member of the Senate Homeland Security and Governmental Affairs Committee. “I can’t help but think, what if the women involved had been spies?. . . It’s such a breach of trust, and it’s virtually unbelievable. I’m truly shocked.

Staff writers Rosalind S. Helderman, Ed O’Keefe, Jason Ukman and Craig Whitlock and special correspondent Juan Forero contributed to this report. Wilson and Forero reported from Cartagena.