News of Michelle Trachtenberg's death prompted tributes from her friends and former "Buffy," "Harriet the Spy," and "Gossip Girl" costars.

Michelle Trachtenberg, famed for roles in Harriet the Spy and Buffy the Vampire Slayer, died at 39. Found unresponsive in her Manhattan apartment, her death is under investigation. Co-star Blake Lively remembered her as a vibrant and dedicated person,

Michelle Trachtenberg was found in her New York City apartment, and the death is not believed to be suspicious, police said.