Sigel, who had been a missionary in Chinatown, was found strangled inside a trunk on 18 June 1909 in the apartment of the prime suspect, a Chinese man named "William" Leon Ling, a waiter in a Chinese restaurant. Sigel had been missing since the 9th of June, when she was last seen leaving her parents' apartment to visit her grandmother.
Sigel's mother taught a Chinese Sunday school class in St. Andrew's Church at 127th Street and Fifth Avenue, while Sigel did missionary work at the Chinatown Rescue Settlement and Recreation Room, reaching out to "American, English, German, French, Hebrew, Italian, [and] Bohemian" girls who had gotten involved with drugs and prostitution. Four years prior to the murder, Leon had kept a chop suey restaurant on Amsterdam Avenue, close to the Sigel home, and Sigel and her mother had first met Leon there during missionary rounds of the local Chinese restaurants. During the murder investigation, 35 love letters signed by Sigel were found in Leon's apartment, along with numerous letters from other women. It was speculated that the motive for murder might have been jealousy, as Chu Gain, manager of the Port Arthur Restaurant on Mott Street, was also found to be in possession of recent love letters from Sigel. Chu reported that he had recently received an anonymous letter threatening Sigel's life if they did not cut off their relationship. Leon was never apprehended, and the murder remains unsolved.
Sigel's murder gained widespread notoriety due to the inter-racial aspects of the relationship as well as the fame of her grandfather, Franz Sigel, who was a U.S. Civil War general. The murder set off a wave of anti-Chinese hysteria, as well as suggestions that the murder was Sigel's own fault.
- "FIND MISS SIGEL DEAD IN TRUNK;" (PDF). The New York Times. June 19, 1909. p. 1. Retrieved 2007-06-16.
- "ARREST FRIEND OF MISS SIGEL;" (PDF). The New York Times. June 20, 1909. p. 1. Retrieved 2009-12-04.
- Woods, Robert Archey; Kennedy, Albert Joseph (1911). Handbook of Settlements. New York: Charities Publication Committee.