New York. Michelle Trachtenberg, a former child star who appeared in the 1996 “Harriet the Spy” hit movie and went on to co-star in two buzzy millennial-era TV shows -- “Buffy the Vampire Slayer” and “Gossip Girl” -- has died. She was 39.

Police responded to a 911 call shortly after 8 a.m. at a 51-story luxury apartment tower in Manhattan’s Hell’s Kitchen neighborhood where officers found Trachtenberg "unconscious and unresponsive,” according to an NYPD statement.

Paramedics pronounced her dead at the scene. No foul play was suspected and the New York Medical Examiner is investigating the cause of death, police said.

“The family requests privacy for their loss," Trachtenberg's representative, Gary Mantoosh, said in a statement Wednesday.

Trachtenberg was 8 when she began playing Nona Mecklenberg on Nickelodeon’s “The Adventures of Pete & Pete” from 1994 to 1996 and then starred in the title role in the film adaptations of “Harriet the Spy” and “Inspector Gadget,” opposite Matthew Broderick.

“Michelle comes off as genuine because she really is a genuine kid. Everyone can identify with her,” said Debby Beece, president of Nickelodeon Movies in 1996.

In 2000 Trachtenberg joined the cast of “Buffy,” playing Dawn Summers, the younger sister of the title character played by Sarah Michelle Gellar between 2000 and 2003.

Trachtenberg thanked Gellar for speaking out against Joss Whedon in 2021, following abuse allegations made against the “Buffy” showrunner. “I am brave enough now as a 35-year-old woman to repost this,” she wrote on social media, and alluded to “his not appropriate behavior” she experienced as a teenage actor.

In 2001, she received a Daytime Emmy nomination for hosting Discovery’s “Truth or Scare.” Trachtenberg went on to recurring roles on “Six Feet Under,” “Weeds” and “Gossip Girl,” where she played the gang’s scheming nemesis, Georgina Sparks.

For her fan-favorite role, she was nominated as a TV villain at the Teen Choice Award in 2012. “It’s definitely a lot more fun than playing the good girl," she told Seventeen in 2009. I love the reaction you get. I never understood why some actors don’t want to play villains or evil characters.”

She was one of the original series' stars to return for a pair of guest appearances in the 2021 “Gossip Girl” revival.

As if to cement herself in millennial culture, Trachtenberg made a cameo in Fall Out Boy’s music video for the “This Ain’t a Scene, It’s an Arms Race” alongside Seth Green.

Hollywood took to social media to mourn one of their own, one who had made the transition from kid star to teen queen to adult actor. “Buffy the Vampire Slayer” co-star David Boreanaz said on Instagram it was “so very sad.. horrible news.” Melissa Gilbert, who starred with Trachtenberg in the 1996 film “A Holiday for Love,” wrote on Instagram: “My heart aches for your family and all those who loved you so.”

Trachtenberg's other credits included “Ice Princess” in 2005, playing a math prodigy and aspiring figure skater. The AP said it had “a good, though feeble, heart and the best of intentions” and said Trachtenberg was “mining the same nervous twitter from her kid-sister days on 'Buffy the Vampire Slayer.'”

The New York City-born Trachtenberg also appeared in the 2004 teen sex comedy “EuroTrip,” she co-starred with Zac Efron and Leslie Mann in 2009's “17 Again” and played a murderous stalker and abductor on an episode of “Criminal Minds.”

For “Killing Kennedy,” the 2013 film in which she played the wife of Lee Harvey Oswald, around 80 percent of Trachtenberg’s dialogue was in Russian. She had learned the language from her mother growing up.

Other credits included supporting roles in the films “Mysterious Skin” in 2004 and “Black Christmas” in 2006. She also starred on the NBC medical series "Mercy" (2009–2010) opposite Taylor Schilling. More recently, she hosted the true-crime docuseries “Meet, Marry, Murder” on Tubi.