Pro-Hamas extremists are flooding social media platforms with calls for attacks on Jewish communities and other targets in the United States and Europe, prompting U.S. law enforcement agencies to step up their readiness postures amid deep concerns about possible violence, American officials and private analysts told NBC News. Tuesday’s explosion at a hospital in Gaza is threatening to become a flashpoint, they said, with posts on X and other platforms portraying it as an Israeli atrocity using an American-made bomb, despite an assessment from U.S. intelligence agencies that the damage resulted from an errant missile fired by a Palestinian militant...