Rosie O’Donnell is mourning the “heartbreaking” loss of her Harriet the Spy costar Michelle Trachtenberg after her sudden death at 39.

In a statement provided to Entertainment Weekly on Wednesday, O’Donnell said of Trachtenberg, “I loved her very much. She struggled the last few years. I wish I could have helped.”

Trachtenberg was found unconscious and unresponsive in her New York City apartment just after 8 a.m. Wednesday, when police and emergency medical workers responded to a 911 call. Authorities said the actress was pronounced dead at the scene, and a medical examiner will determine the cause of death. Criminality is not suspected, and the investigation is ongoing.

O’Donnell, 62, played the nanny and confidant to Trachtenberg’s clever kid detective in Harriet the Spy, which hit theaters in 1996 and marked Trachtenberg’s first major film role. Until then, the young actress had been known for her work on TV’s The Adventures of Pete & Pete and All My Children.

Reflecting on Harriet the Spy in 2021, Trachtenberg spoke fondly of the experience and working with O’Donnell.

“I turned 10 years old on the first day of principal photography of Harriet this Spy,” she told Entertainment Tonight. “Rosie was my biggest supporter. There was a lot required of me. I’m extremely grateful for the experience.”

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After Harriet, Trachtenberg went on to play notable roles on Buffy the Vampire Slayer, as little sister Dawn Summers, and Gossip Girl, as chaos agent Georgina Sparks. Trachtenberg would also reprise the latter role for the short-lived Gossip Girl reboot.

Trachtenberg’s death has been met with an outpouring of love and grief from colleagues and fans alike. 

Gossip Girl alum Ed Westwick wrote on social media that he was “so sad to hear” of her passing, adding that he was “sending prayers.”

Fellow Nickelodeon alum Kenan Thompson paid tribute to Trachtenberg as well, writing, “Our first Nick movie star has departed us!! She was my friend and now she rests! Check on your people!!!”