The more that Americans learn about Pope Leo XIV and all his complex, diverse family history, the more they see themselves.

One set of genealogists uncovered that on his mother’s side of the family, Pope Leo descended from Creole people of color from New Orleans, a discovery that thrilled residents of that city who felt a connection to the first American pope.

Now, another story on a different side of the pope’s family has emerged. Genealogists have found that his paternal grandparents in Chicago were once accused of a charge that might seem prudish now: A century before Robert Prevost became the pope, his grandparents were arrested for “unbecoming conduct,” a story that was breathlessly covered in the gossipy, sensationalist newspapers of the late 1910s.

The press called it “an illicit affair” that had morphed into a love triangle. Newspaper articles and other public records show that the pope’s grandfather, Salvatore Giovanni Riggitano, was married to another woman while accused of having a relationship with Suzanne Fontaine, who later became the pope’s grandmother. Mr. Riggitano and Ms. Fontaine would eventually take the surname Prevost from Ms. Fontaine’s mother, starting a new family whose lineage now leads directly to Vatican City.

The story of Pope Leo’s grandparents, European immigrants who found love and a brush with the law in Chicago, is incomplete and somewhat murky. But public records give a glimpse into what happened, beginning in their home countries of Italy and France, and eventually tracing to the pope, who will be formally installed as pontiff in Rome on Sunday.

A group of online genealogists, the Genealogy Discord, released a report on Thursday revealing the background of the pope’s grandparents and how their names and lives shifted over time.