The nonprofit organization Secure Community Network, which provides safety consulting and training for Jewish facilities across North America, wrote a letter on Monday to Elon Musk, Twitter’s new owner, imploring him to clamp down on antisemitic content that could endanger lives. “Twitter has an antisemitism problem — with hashtags such as #holohoax [Holocaust Hoax] and #killthejews abounding on the site,” Michael Masters, the group’s national director and chief executive, wrote in the letter. The group is made up of many former law enforcement officials, including Mr. Masters, who was the chief of staff at the Chicago Police Department. It gave...