A member of the violent Mara Salvatrucha street gang was convicted in federal court Thursday of prostituting a 12-year-old girl who was a runaway, authorities said.

Jose C. Juarez-Santamaria, 24, met the girl at a Halloween party in Oxon Hill in 2009, according to court papers. The girl asked for his help finding a place to stay, but he began prostituting her the next day.

“From the moment he laid eyes on the victim, Juarez-Santamaria did not see a young 12-year-old runaway in need of help,” Neil MacBride, U.S. attorney for the Eastern District of Virginia, said in a statement. “He saw a money-making opportunity.”

Gang members gave her clothes, makeup and condoms, according to court papers. Prosecutors said Juarez-Santamaria, known as Sniper, and others gave her alcohol and marijuana.

During a trial in U.S. District Court in Alexandria that was presided over by Judge Liam O’Grady, the girl testified that Juarez-Santamaria and other gang members called potential customers, authorities said. She said she was taken to have sex with them at businesses, homes and hotels in the Northern Virginia area, sometimes every day during a week.

Juarez-Santamaria generally charged people $40 for fifteen minutes of sex with the girl, authorities said, but gang members had sex with her for no charge. Sometimes customers lined up for her.

Juarez-Santamaria, an illegal immigrant from El Salvador, was the leader of a Mara Salvatrucha clique, officials said. The gang, also known as MS-13, originated in Los Angeles and is known for being violent.

David T. Williams, defense attorney for Juarez-Santamaria, said in an e-mail Thursday that his client is “considering his options” but declined to comment further.

Juarez-Santamaria was convicted of conspiracy, sex trafficking and transportation of a minor for prostitution, officials said. He is to be sentenced Oct. 28 and faces 15 years to life in prison.

Early this month, another MS-13 member, Alexander Rivas, 19, of Alexandria, was sentenced to 10 years in prison for trafficking two juvenile females for sex, prosecutors said. They said Rivas found the girls customers throughout Northern Virginia.