Her daughter had been missing for about a month when Maria Reyes hacked into the 15-year-old’s Facebook account and discovered the death threats.

“Those guys want to kill you,” warned one private message in Spanish. “They have already given permission to take you out.”

“Get ready,” said another.

The profiles of the people making the threats featured skulls, guns, coffins and gang signs that Reyes immediately recognized as emblems of Mara Salvatrucha, better known as MS-13. The 36-year-old had witnessed the street gang’s brutality in El Salvador. And in the summer of 2014, she had sent for her daughter to join her in the United States to escape MS-13.

Now she looked at the threats and realized that she had lost her daughter to the gang after all.

Four weeks later, on Feb. 11, police found Damaris Alexandra Reyes Rivas’s body near an industrial park in Springfield, Va. She had been dead for abouta month.

Police have arrested 10 people in connection to the gang-related killing, as well as six more for a related slaying — a sobering sign of MS-13’s resurgence in the Washington region.

The rise in MS-13-related violence comes at a time of intense national debate over immigration and law enforcement.