Blake Lively and Alyson Hannigan are among the former "Gossip Girl" and "Buffy" actors mourning the death of their one-time co-star, Michelle Trachtenberg.

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

Blake Lively paid tribute to her Gossip Girl costar Michelle Trachtenberg. Yesterday, news emerged that the actor had died at age 39. Though her cause of death is still unknown, police sources noted that she recently underwent a liver transplant.