A spokesman for the Anti-Defamation League said that before Saturday’s shooting, the deadliest anti-Semitic attack in recent United States history was in 1985, when a man killed a family of four in Seattle. He had mistakenly thought they were Jewish.

[The 11 people killed in Pittsburgh were remembered as guardians of their faith. Read more about their lives here.]

There was also an attack by a white supremacist on a Jewish Community Center filled with children in Los Angeles in 1999 that injured five. More recently, in 2014, a white supremacist opened fire outside a Jewish Community Center in a suburb of Kansas City, Mo., killing three people.

“I’m not a Chicken Little who’s always yelling, ‘It’s worse than it’s ever been!’ But now I think it’s worse than it’s ever been,” said Deborah E. Lipstadt, a professor of Holocaust history at Emory University in Atlanta and the author of a planned book on anti-Semitism.

Ms. Lipstadt said she did not wish to be seen as alarmist, because in some ways “things have never been better” for Jews in America.

But she likened anti-Semitism to a herpes infection that lies dormant and re-emerges at times of stress. It does not go away, no matter how “acculturated” Jews have become in America, because “it’s a conspiracy theory,” said Ms. Lipstadt, whose win at trial against a Holocaust denier in England was portrayed in the 2016 movie “Denial.”