Michelle Trachtenberg tributes: Blake Lively, Kenan Thompson, more stars mourn actress

Michelle Trachtenberg's famous co-stars, from Blake Lively to Rosie O'Donnell, are paying tribute after the actress's death on Wednesday.

The 2000s mainstay, who boasted memorable roles in "Gossip Girl," "Harriet the Spy" and "Buffy the Vampire Slayer," was found dead Wednesday morning in New York City, a spokesperson for the New York Police Department confirmed to USA TODAY. A cause of death has not been disclosed.

Lively, who played Serena van der Woodsen opposite Trachtenberg's Georgina Sparks, penned an emotional tribute on her Instagram Story Wednesday. "She was electricity. You knew when she entered a room because the vibration changed. Everything she did, she did 200%," she wrote.

"The world lost a deeply sensitive and good person in Michelle," Lively continued. "May her work and her huge heart be remembered by those who were lucky enough to experience her fire."

O'Donnell, who co-starred with Trachtenberg in "Harriet the Spy," told Us Weekly that Trachtenberg's death was "heartbreaking," adding, "I loved her very much. She struggled the last few years. I wish I could have helped."

See the other stars who are paying tribute to Trachtenberg.

Michelle Trachtenberg dies: 'Gossip Girl' and 'Harriet the Spy' actress was 39

Kim Cattrall, Kenan Thompson remember 'sweet' Michelle Trachtenberg

Trachtenberg's "Buffy" co-star David Boreanaz posted her death announcement to his Instagram Story, writing, "So very sad …horrible news. RIP and prayers to her and her family."

Kim Cattrall, who performed alongside Trachtenberg in the Disney sport drama "Ice Princess," shared a heartfelt photo from the beloved 2005 film. "Rest in peace sweet Michelle ??,” Cattrall wrote.

"Little House on the Prairie" alum Melissa Gilbert, who shared the screen with Trachtenberg in the 1996 TV movie "A Holiday for Love," mourned her loss in an Instagram post.

"Oh Michy…and we lived so close to one another….." Gilbert said. "My heart aches for your family and all those who loved you so…"

Alyson Hannigan, Trachtenberg's co-star on "Buffy the Vampire Slayer," also took to Instagram to pay tribute to the late actress.

"I am deeply saddened by the news of Michelle's passing," wrote Hannigan alongside a series of photos of the pair. "She brought a loving energy to the set of Buffy. My thoughts are with Michelle's family and friends."

Former Nickelodeon star Kenan Thompson shared a throwback clip of Trachtenberg on Instagram, writing, "Our first Nick movie star has departed us!! She was my friend and now she rests!! ???? Check on your people!!! ??"

Michelle Trachtenberg's memorable roles: From 'Gossip Girl' to 'Buffy'

'Gossip Girl' creators mourn Michelle Trachtenberg's death

In a joint statement to USA TODAY, "Gossip Girl" creators Josh Schwartz and Stephanie Savage said they are "deeply saddened by the news of Michelle's passing," adding the actress was "so funny, kind and talented."

"Her portrayal of Georgina Sparks was an iconic fan favorite and grew from a one season villain to a beloved character who returned over the course of six seasons," Schwartz and Savage continued. "She was a delight to have on set and will be deeply missed. Our thoughts are with her family."

Trachtenberg's "Gossip Girl" co-star Ed Westwick, who played Chuck Bass, mourned his former castmate on his Instagram Story, writing, "So sad to hear of the passing of @michelletrachtenberg sending prayers ??????"

Meanwhile, Trachtenberg's "Buffy" co-star James Marsters paid tribute to her in a heartfelt Facebook post.

"My heart is heavy today. We have lost a beautiful soul. Michelle was fiercely intelligent, howlingly funny, and a very talented person. She died much too young, and leaves behind scores of people who knew and loved her," Marsters wrote.

He continued: "My heart goes out to her family who are good people, and are suffering the greatest loss anyone could bear. I hope everyone can give them space to heal in this most difficult time. Godspeed Michelle. You are missed."

(This story was updated to add new information.)

Contributing: Edward Segarra, USA TODAY

This article originally appeared on USA TODAY: Michelle Trachtenberg dies: 'Gossip Girl,' 'Buffy' co-stars react