The killing of Americans in a Mexican border city highlights a sinister trend that US policies are making worse

• Four Americans were abducted in Mexico this month in an incident that left two of them and a bystander dead.
• The city where the incident happened, Matamoros, is a gathering place for migrants seeking to enter the US.
• The incident and others like it highlight the vulnerability that thousands of migrants now face.

The March 3 abduction of four Americans in the Mexican city of Matamoros, just across the border from Brownsville, Texas, was not the first such crime by Grupo Escorpiones, an armed branch of the Gulf Cartel.