More than 9,000 former and current Border Patrol agents were part of a private Facebook group that joked about migrants’ deaths and targeted members of Congress, including Representative Alexandria Ocasio-Cortez (D-N.Y.). The secret Facebook group came to light as members of Congress visited a detention facility in Texas on July 1. The Facebook group, called “I’m 10-15,” was created in August 2016 and included about 9,500 members from across the US, according to ProPublica who first reported about the postings. ProPublica said that it was able to link the posts to legitimate Facebook profiles,...