Investigators found bits of flesh inside the bedroom, and part of Kasian's scalp and her ear were found in a trash bin at the bottom of the garbage chute outside the home, NBC reported.
The Los Angeles Department of the Medical Examiner ruled that the woman's cause of death was exsanguination, the act of draining a person of blood.
Leibel, 37, had tried to keep police out of the apartment by barricading the door.
During Leibel's trial, pathologists testified the horrific murder set a precedent in terms of the brutality and grisliness of Kasian's injuries.
Graphic images of the scene and of Kasian were shown to the jury, as were images from a graphic novel called Syndrome, which Leibel helped to create in 2010.
In the novel, a scientist tries to find a way to cure people of evil. One page displays a woman, lying naked on a bed with no head.
The novel begins with the foreword: "If you loved hurting things, what would you do?" and ends with an image of a hand dripping blood and the caption: "In the end, we all become monsters".
Prosecutors District Attorney Tannaz Mokayef and her co-counsel, Deputy DA Beth Silverman argued the 37-year-old had mirrored elements of the novel in Kasian's murder, and that the book was the man's "blueprint".
Silverman told the court there were no words to describe the depravity of Leibel's actions and the extremity of the violence.
"This was depravity. Ms. Kasian died a slow and painful death," she said.
"He threw away pieces of his fiancée like she was trash. The mother of his newborn baby.
"Her daughter will never know her, and that's because of the unconscionable acts you heard about during this trial."
Leibel had pleaded not guilty to murder, mayhem, aggravated mayhem and torture.
When the jury read out their sentence on Wednesday, Kasian's mother, a Ukrainian health care worker named Olga, broke down in tears.
Leibel's reaction was one of an unmoved man. He stared straight ahead and did not move.
He will be sentenced on June 26, and faces life in prison without the possibility of parole.
Though he was earlier facing the death penalty, the LA County District Attorney's Office had earlier opted not to seek it against him.