“This American style of gangsterism was imported like a catalyst” and became mixed with the local gangs into a violent and murderous mash-up, Valdez said. “It took off like a wildfire, like a virus out of control.”

The most high-profile are rivals MS-13, formally known as “Mara Salvatrucha,” and the 18th Street gang, or M-18, both of which have roots in the U.S. It was in Los Angeles where Mexican immigrants formed the 18th Street gang, and then began recruiting Central American immigrants in the 1980s. MS-13 was made up of mostly Salvadoran immigrants who were trying to protect themselves from other gangs, Valdez said.

They effectively assimilated into the American gang lifestyle, getting tattoos, wearing certain clothes and learning how to recruit. But when the U.S. decided to clamp down on undocumented immigrants with rap sheets, they were sent back to Central America armed with what they learned in the U.S., said Magdaleno Rose-Avila, a Seattle immigration activist who has studied gangs in El Salvador.