Anti-Semitism was supposed to be a disease of the far right. But the people actually killing Jews in France these days are not members of the National Front. They are Islamists.

“The major crimes against the Jewish community — Ilan Halimi, the Toulouse killings, the Hyper Cacher killings, Sarah Halimi — all of them have all been carried out by radicalized Muslims,” Robert Ejnes, the executive director of CRIF, an umbrella organization of French Jewish groups, told me in a call from Paris. “These young people have French identity cards, but they hate what France stands for. This is the nature of the problem we are facing. And it’s very hard to talk about.”

Here are some facts that are very hard to talk about: Jews represent less than 1 percent of the population in France, yet in 2014, 51 percent of all racist attacks were carried out against them, according to the French Interior Ministry. A survey from that year of about 1,000 French respondents with unknown religious affiliation and 575 self-identified Muslims, conducted by the AJC Paris and the French think tank Fondapol, found that the Muslim respondents were two or three times more likely to have anti-Jewish sentiments than those from the random French group. Nineteen percent of all respondents felt that Jews had “too much” political power. Among Muslims, the number was 51 percent. As for the idea that Zionism “is an international organization that aims to influence the world and society in favor of the Jews,” 44 percent of Muslims surveyed approved of this statement. The rest of the survey is just as devastating.

For years now, France has deployed armed troops to protect Jewish synagogues and schools. But the violence on the streets — a 15-year-old girl wearing the uniform of her Jewish school slashed in the face; an 8-year-old boy wearing a kippah assaulted; teenage siblings called “dirty Jews” before being beaten — hasn’t abated. On Wednesday, hours before a march in honor of Mireille Knoll, the office of the Union of French Jewish Students at the Sorbonne was ransacked and defaced with graffiti like “Viva Arafat” and “death to Israel.”

Whatever else the investigation of Ms. Knoll’s murder might reveal, this much we know for certain: The men who are accused of killing her were living in a culture in which Jews are reviled on the far right and, increasingly, on the far left; in which sensitivity toward cultural differences have driven too many for too long to ignore the spread of an ancient hatred in a vicious new form; in which attacks on Jews have been explained away as politically motivated by events in the Middle East. In such a culture, it shouldn’t come as a surprise that some would come to the conclusion that Jewish blood is cheap.

In the wake of Ms. Knoll’s murder, all of the usual lines are being repeated. Anti-Semitism is the hatred that never dies. Violence that begins with the Jews never ends with them.

All of this is true. What’s also true is that anti-Semitism is the oldest hatred in the world because individual people have sustained it in every generation. It cannot be defeated until we look these people and their ideologies in the face.

Every French Jew — like millions of Jews throughout history — will have to make their own choice about whether to leave their homes for safer shores or to stay and fight for their rightful place in a country that prides itself on being a beacon of liberty and fraternity. But perhaps the better part of wisdom is with one of Mireille Knoll’s granddaughters, Noa Goldfarb. Following her grandmother’s murder, she wrote in a Facebook post from Israel: “Twenty years ago, I left Paris knowing that neither my future nor that of the Jewish People is to be found there.”