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, who was known as “Sniper,” met the girl at a Halloween party in Oxon Hill in 2009, according to court papers. The girl asked for his help in finding a place to stay, according to authorities, 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, the U.S. attorney for the Eastern District of Virginia said in a statement. “He saw a money-making opportunity.”

Juarez-Santamaria, who led a clique of the gang Mara Salvatrucha, generally charged people $40 for fifteen minutes of sex with the girl, authorities said, but gang members had sex with her for no charge. Juarez-Santamaria and others gave her alcohol and marijuana, according to authorities.

Juarez-Santamaria is an illegal immigrant from El Salvador, authorities said. A jury in U.S. District Court in Alexandria convicted him of conspiracy, sex trafficking and transportation of a minor for prostitution, officials said.

Juarez-Santamaria faces 15 years to life in prison, and is scheduled to be sentenced Oct. 28, authorities said.

David T. Williams, Juarez-Santamaria’s defense lawyer, said in an e-mail Thursday that his client is “considering his options” and declined further comment.