After the attack on Tree of Life, Rabbi Marvin Hier, the founder and dean of Simon Wiesenthal Center, told The Times, “I’m afraid to say that we may be at the beginning of what has happened to Europe, the consistent anti-Semitic attacks.”

“If it is not nipped in the bud,” he said, in a remark that should make every American pause and think, “I am afraid the worst is yet to come.”

Anti-Semitic claims have acquired new energy online, to the point that Mark Zuckerberg, the founder of Facebook, recently cited one — that the Holocaust never actually happened — as an example of offensive but good-faith argument on his social-media platform. “I think there are things that different people get wrong,” Mr. Zuckerberg said in an interview with the journalist Kara Swisher. “I don’t think that they’re intentionally getting it wrong.” He subsequently clarified that he “absolutely didn’t intend to defend the intent of people who deny” that, within living memory, Hitler’s Germany killed six million Jews as part of a systematic campaign to kill them all.

The suspect in the Pittsburgh killings, Robert Bowers, had found a home for his hate on Gab, a new social network that bills itself as a guardian of free speech, unlike somewhat less permissive platforms like Twitter. There his online biography read, “Jews are the children of Satan,” a statement of personal values that he evidently expected to earn him not opprobrium but followers.

Alongside anti-Semitism, anti-black hatred appears to be rising. It has been expressed recently not only in incidents in which white Americans have harassed black Americans for gardening, coming home, swimming, working or campaigning for public office, but in deadly attacks like the one by a bigot who shot two black people at a Kentucky grocery last week, after he tried but failed to enter a black church.