Rosie O’Donnell paid tribute to her former co-star, Michelle Trachtenberg, who was found dead in her New York City apartment on Wednesday.

“Heartbreaking,” O’Donnell said in a statement to Us Weekly. “I loved her very much.”

As of Wednesday, Trachtenberg’s cause of death was unknown. Police sources told ABC News that the actor had undergone “a liver transplant and may have been experiencing complications.”

O’Donnell said in her statement that Trachtenberg “struggled the last few years,” adding, “I wish I could have helped.”

The two actors appeared together in Trachtenberg’s onscreen debut in the 1996 film, “Harriet the Spy.”

New York City police told HuffPost that “criminality is not suspected” in the death.

“The Medical Examiner will determine the cause of death,” the NYPD said. “The investigation remains ongoing.”

Trachtenberg is also known for her roles in the TV series “Buffy the Vampire Slayer” and “Gossip Girl.”