Montgomery County police arrested four suspected gang members Monday night after walking in on a robbery at a brothel operated out of a Wheaton apartment, authorities said yesterday.

Police went to the apartment, in the 2300 block of Blueridge Avenue, looking for a carjacking suspect, according to a police document.

"Upon arrival and knocking on the door a voice stated, 'come in,' " Detective David Woolsey wrote. "The officers and detectives entered the apartment and bedlam ensued."

Four males, who officers believe belong to Mara Salvatrucha, ran toward the back of the apartment, where police found three men lying on the floor with their hands tied behind their backs, authorities said. Inside a room they found a 24-year-old prostitute who had been sexually assaulted by the suspects, police said.

Their arrests come a week after the fatal roadside shooting of a Guatemalan immigrant in Silver Spring in which two other suspected members of Mara Salvatrucha, or MS-13, were charged.

Montgomery County Assistant State's Attorney Jeffrey Wennar said the six men are the first suspected MS-13 gang members arrested in the county since the Aug. 5 stabbings at Springbrook High School in which eight suspected gang members were arrested. Four other suspected MS-13 members accused in another stabbing were arrested later that day at a Target store in Wheaton.

He said the recent cases do not necessarily indicate an increase in gang violence in the county.

"I think that their violence has been steady, and it's not an indication of increase or decrease. It was a crime that was interrupted and thwarted by police," said Wennar, a gang expert. "This is a continuing pattern of the type of behavior that gang members engage in."

Officers arrested Jovet Ivarroca, 17, of Gaithersburg, who was charged as an adult; Juan C. Hernandez-Jimenez, 20, of Beltsville; and Jose Domingo Hernandez, 20, and Victor Ramirez, 20, whose addresses were unknown. All four suspects were charged with armed robbery and first-degree rape.

According to the police document, the suspects were robbing the brothel's proprietor and the prostitute when two clients walked in.

The two clients were robbed at gunpoint with a .38-caliber Rossi handgun, according to the document. The suspects stole $256 in cash from the three men, police said. The victims' hands were tied, and they were placed on the floor.

Police said Ivarroca forced the woman to perform oral sex on him. Hernandez-Jimenez and Domingo Hernandez had intercourse with her against her will, police said.

Katherine Chon, an expert on human trafficking and prostitution in the Washington area, said brothels in the region have adopted a lower profile in recent years as police have targeted those that operate more openly as massage parlors and spas.

She said establishments such as the one in Wheaton -- described by police as an "unlicensed cantina" -- are attractive to robbers because owners deal primarily in cash and customers might be reluctant to call police when they have been the victims of a crime.

"It's an attractive place" for robbers, said Chon, co-executive director of Polaris Project, an organization that monitors human trafficking and coerced prostitution in the Washington area. "It's easier to get away with it when you're committing a crime against another criminal."

Staff researcher Bobbye Pratt contributed to this report.