A second private autopsy, requested by Smith's family, also failed to find what had killed the girl.
During this time, Smith posted frequently about her dead daughter on Instagram, as well as her struggles with mental illness.
In one post six months after Zadie's death, Smith wrote that "life was unfair" as "it took you too soon".
"My heart is breaking all over again. It happens every day I wake up and remember she's gone," she wrote in the caption.
"I am so lost without you. rest in peace, sweet angel. mommy loves you forever."
Another post by Smith on April 7, 2017, the mum wrote that she was "completely lost" without Zadie and she had a "broken and empty heart".
On April 13, less than a week later, Smith walked into a police station and confessed to killing Zadie.
Athens Police chief Floyd Johnson told reporters shortly after Smith's confession she had been struggling with mental health issues and had been off her medication at the time her daughter had been killed.
An emotional Johnson said Zadie's death and Smith's mental health issues made the case tragic for all involved.
"[She] told us that she has mental issues and, at the time she had some issues and she was not on her medication," he said, according to Fox 30.
"She went into Zadie's room, put a pillow over her head; her face – she was sleeping on her back – and smothered the child.
"A short time later, she realised what was going on; she removed the pillow. Zadie was not breathing, and she called 911."
Smith hadn't confessed at the time of her daughter's death because she was afraid of the consequences but her conscience had later got the better of her, Johnson said.
"Yesterday, it was very clear [that] she was in a very good frame of mind," he said.
"She knew exactly what she was doing. She was talking with us. She wanted to get this out, that she had caused this."
Smith's trial had been repeatedly delayed because of coronavirus before she decided to plead guilty to a lesser charge of felony murder.
She was sentenced on Monday to a life sentence with the possibility of parole.
WHERE TO GET HELP:
If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.
OR IF YOU NEED TO TALK TO SOMEONE ELSE:
• 0800 543 354 (0800 LIFELINE) or free text 4357 (HELP) (available 24/7)
• https://www.lifeline.org.nz/services/suicide-crisis-helpline
• YOUTHLINE: 0800 376 633
• NEED TO TALK? Free call or text 1737 (available 24/7)
• KIDSLINE: 0800 543 754 (available 24/7)
• WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757 or TEXT 4202
