Her husband, Justin Ross Harris
, the child's father, who is in jail without bond, has also told police that he used the Internet to research child deaths in vehicles and what temperature it needs to be for death to occur, police said.
"Justin stated that he was fearful that this could happen," the police affidavit said.
In the document released Sunday, police say that during questioning Leanna Harris "made similar statements regarding researching in car deaths and how it occurs."
The time frame for when this alleged research took place remains unclear.
Their son, Cooper Harris, who was 22 months old when he died June 18, was buried after a funeral Saturday in Tuscaloosa, Alabama.
Leanna Harris emphatically told a crowd at her son's funeral she loves and stands by her husband, even though he is charged with murder in the child's death.
"Am I angry with Ross?" Leanna Harris said Saturday. "Absolutely not. It has never crossed my mind. Ross is and was and will be, if we have more children, a wonderful father. Ross is a wonderful daddy and leader for our household. Cooper meant the world to him."
Police said Justin Ross Harris told them he forgot to drop his 22-month-old son at the day care center before going to work. The boy died after spending seven hours in a child safety seat in the back of an SUV.
Harris was arrested hours after his son's death and has pleaded not guilty to charges of murder and second-degree child cruelty.
The affidavit was filed in the application for a search warrant used to seize computers from the Harris home.
A timeline of events
On the day Cooper died, his father stopped for breakfast at a fast-food restaurant and afterward strapped his son into a rear-facing child restraint seat on his SUV's back seat, police said.
He drove to his workplace, a Home Depot corporate office, about a half-mile away. He works as a Web designer there.
Usually, he would take his son to an on-site day care. But that day, police said, Harris left him in the car seat.
During his lunch break, he returned to his car, opening the driver's side door to put something inside, police said.
After work, around 4:16 p.m., the 33-year-old father got in his car and drove away. A few miles away, he stopped the car at a shopping center and called for help.
When it became clear Cooper was dead, Harris was so inconsolable police had to restrain him.
"What have I done?" he wailed as he tried to resuscitate the boy.
The Cobb County medical examiner's office found the child's cause of death "consistent with hyperthermia and the investigative information suggests the manner of death is homicide," according to a Cobb County Department of Public Safety statement issued last week. Temperatures hit 92 degrees Fahrenheit on the day of Cooper's death.
The medical examiner's office is waiting for toxicology test results before making an official ruling as to the cause and manner of the toddler's death.