Michelle Trachtenberg, the “Buffy the Vampire Slayer” and “Gossip Girl” actress who was found dead in a Manhattan apartment in February, died from complications of diabetes, the New York City medical examiner’s office said on Wednesday.

The medical examiner’s office initially said that the cause and manner of Ms. Trachtenberg’s death was “undetermined.” The office amended that conclusion this week after reviewing toxicology test results, a spokeswoman Julie Bolcer, said on Wednesday, ruling that she died a natural death from complications of diabetes mellitus.

Officers with the New York Police Department found Ms. Trachtenberg, 39, unconscious and unresponsive in the apartment after responding to a 911 call just after 8 a.m. on Feb. 26. She was pronounced dead by emergency medical workers, who had also responded, the authorities said at the time.

No autopsy was performed because of religious objections from Ms. Trachtenberg’s family, Ms. Bolcer said.

Ms. Trachtenberg began acting in commercials at age 3. Before she was 10, she was making regular appearances in television shows, including the Nickelodeon sitcom “The Adventures of Pete & Pete” and the ABC daytime drama “All My Children.”

At 11, Ms. Trachtenberg landed the title role in the film “Harriet the Spy” (1996). She drew praise for her performance as Harriet Welsch, the precocious girl who draws scorn from schoolmates over her imagined spy adventures, which she meticulously chronicles in a notebook.

She found a new level of fame as a troubled teenager on the supernatural 1990s series “Buffy the Vampire Slayer” and as a conniving young socialite on “Gossip Girl.”