In the ’90s and still today, United States authorities rounded up these gang members and deported them back to El Salvador. From there, the new seeds of gang culture, imported from the United States, grew into the vicious networks that operate with impunity throughout El Salvador.

“The U.S. has such a deep impact on El Salvador for its own foreign policy needs that it bears significant responsibility, not only for the flow of migrants out of El Salvador into the U.S., but also for the current conditions of violence that exist there,” said Erik Ching, a history professor at Furman University in South Carolina.

El Salvador has been struggling beneath the weight of these deportees and the turbulent dynamic they brought with them. Prisons are overflowing and have become cesspools of disease and overcrowding. On the outside, joblessness and violence are the broken realities of life.

The potential for 200,000 others to return will only exacerbate those problems. And in the meantime, the exodus continues: More than 250 Salvadorans leave the country each day, said Mr. López of Cofamide.

Citing survey and medical data released last year that was gathered by its teams working with Salvadoran migrants traveling through Mexico, Doctors Without Borders said 55 percent of Salvadoran refugees and migrants reported being victims of blackmail or extortion. An additional 56 percent had a relative lost to violence, and 67 percent said they never felt safe at home.

The Border Patrol apprehended more than 27,000 families in the year ending in September 2016, and an additional 17,500 children made the trip alone. And those figures were down from recent years.

“Thousands of families will be divided, and those who stay in the U.S. will be desperately worried about the safety of their relatives going back to a poor, unprepared country where their physical safety will be threatened every single day,” said Mr. Lindo-Fuentes of Fordham.