In the early-morning hours of April 27, 1913, a night watchman at a pencil factory in Atlanta was startled when his lantern shone upon the mutilated, lifeless body of a girl who had been in the company’s employ. She was 13-year-old Mary Phagan, and her murder became one of the most sensational crimes of the era. It also sparked what has been described as perhaps the most vicious episode of anti-Semitism in U.S. history: the lynching of Leo Frank, the factory’s Jewish superintendent.

The historian Leonard Dinnerstein, who has died at 84, distinguished himself as an authority on the event with his doctoral dissertation, first published as “The Leo Frank Case” in 1968. His subsequent volumes, among them “Anti-Semitism in America” (1994), established him more broadly as a scholar of that ancient prejudice — its roots, its flare-ups and the ways in which it has been overcome, or not, in modern-day America.

Dr. Dinnerstein, a professor for more than 30 years at the University of Arizona, where he helped build the Judaic studies program, died Jan. 22 at his home in Tucson. The cause was renal failure, said his daughter, Julie Dinnerstein.

Dr. Dinnerstein, the son and grandson of Jewish immigrants from Eastern Europe, said he grew up “on the cusp” — after the generations of Jews who fled to America from the pogroms and before those for whom such events existed only in the distant past.

“The older the person is, the more likely he or she will believe that the Jew is other, that he is outside the mainstream and that he can’t trust gentiles,” the Jewish News of Greater Phoenix quoted him as saying at a 1998 symposium. He described Jews as existing “on the edges of Christian society,” endeavoring to “do the right things to make them think well of us.”

He said he stumbled unwittingly into the Frank case as a doctoral student. He was considering devoting his dissertation to the ongoing civil rights movement but felt his curiosity piqued when an acquaintance remarked to him, “Remember, the Jews were involved in civil rights before it became a Negro issue.”

A friend who was writing about the American Jewish Committee, an advocacy group founded in 1906, suggested that he pursue the Frank case.

“At the time I thought that I was exposing an injustice that occurred in the United States during the Progressive era,” he reflected years later, in a 2008 preface to a revised version of his work. “I simply hoped [it] would be published; I had no way of knowing the impact that it would have on scores of students and general readers as well as the contribution it would provide to the development of a more professional analysis of the American, and especially the Southern, Jewish past.”

Phagan’s murder, Dr. Dinnerstein wrote, fanned the embers of Southern resentment at the increasingly industrialized economy, the exploitation of workers and the Northerners whom they perceived as responsible for it. Despite meager evidence brought against him at his trial — where crowds gathered outside shouting “Kill the Jew” — Frank was sentenced to death.

In 1915, after the governor commuted his sentence to life imprisonment, a mob proclaiming itself the Knights of Mary Phagan abducted Frank from prison and hanged him from a tree. Half of Georgia’s 3,000 Jews fled the state, the New York Times reported years later.

In 1982, a witness came forward to accuse another factory employee as Phagan’s killer. Four years later, while not asserting Mr. Frank’s innocence, Georgia pardoned Frank because the state had been unable to ensure his safety and had failed to prosecute his killers.

Dr. Dinnerstein conducted a broader historical study in “Anti-Semitism in America,” a volume that the journalist and writer George Packer described as an account of “one of the country’s dirty secrets.”

In the volume, Packer wrote in a Chicago Tribune review, Dr. Dinnerstein “cataloged three centuries of American anti-Semitism of every kind, from social slights in elite clubs to dubious racial theorizing by respectable scholars, from beatings by Irish cops in 1902 to fraternity pranks in 1978, from Father Coughlin to Minister Farrakhan.”

Anti-Semitism reached its apex in the United States in the early 1940s, Dr. Dinnerstein wrote, as Americans emerged from the Depression and feared new rounds of immigration amid World War II. He described one alarmist speaker stoking fears about “200,000 Communist Jews at the Mexican border waiting to get into this country,” ready to “rape every woman and child that is left unprotected.”

In his view, anti-Semitism was too deeply embedded in Christian culture to ever completely fade from American life. But through efforts such as the Second Vatican Council in the 1960s, which rejected anti-Semitism and age-old depictions of Jews as the killers of Christ, he found room for hope, citing his own experience as evidence.

“When I do something people respond to me as an individual,” he said at the 1998 event, “not as a member of a group.”

Leonard Dinnerstein was born in the Bronx on May 5, 1934. His mother, the daughter of immigrants from Austria and Romania, was a homemaker. His father, who was born in modern-day Belarus, was a postal employee before working for his wife’s family’s grocery during the Depression.

Dr. Dinnerstein received a bachelor’s degree in history from the City College of New York in 1955 before studying American history at Columbia University, where he received a master’s degree in 1960 and a PhD in 1966. He joined the University of Arizona in 1970 and retired in 2004.

Survivors include his wife of 57 years, the former Myra Rosenberg of Tucson; two children, Julie Dinnerstein of Manhattan and Andrew Dinnerstein of Scottsdale, Ariz.; a sister; and a granddaughter.

Dr. Dinnerstein’s books included “Ethnic Americans” (1975), co-authored with David M. Reimers; “Natives and Strangers” (1979), co-authored with Reimers and Roger L. Nichols; and “America and the Survivors of the Holocaust” (1982). “Anti-Semitism in America” received a National Jewish Book Award.

As for Leo Frank, “the story,” Dr. Dinnerstein wrote in 2008, was “not a pretty one. As we continue to learn more about the murder of Mary Phagan, there are still people who sincerely believe that Frank was guilty of the crime for which he was convicted. Some are still in doubt. I have no doubts: Frank was innocent.”