The bodies of 15-month-old Nicholas Crowder and 32-year-old Tiffany Spears were discovered at their home in Tennessee in February this year.
An autopsy has now found Spears died from acute combined drug toxicity from methampheamine and fentanyl, according to local media.
Meanwhile, Nicholas - who was found in his car seat on a bedroom floor - died from starvation and dehydration, his autopsy reported ruled.
Authorities said he had been strapped in without access to food or water, according to local broadcaster WKRN News 2.
His sister seemed to try and take care of her younger brother for several days, The Tennessean reported.
Food - including dried eggs - were reportedly found near the car seat.
Both deaths were ruled as accidental, according to local media.
The Sumner County Sheriff’s Office said officers were called to the scene by a probation officer making a home visit at the residence in Portland, a city in Tennessee.
According to Spears’ autopsy, a probation officer raised the alarm after seeing the 32-year-old mother lying still on the ground with a three-year-old running around, The Tennessean reported.
Police reportedly found the mother motionless on the bathroom floor with no obvious signs of trauma.
The autopsy ruled she had been dead for several days, according to the local newspaper.
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