Leanne Harris, 30, has not yet been charged with any crime, but according to reports there are suspicions about what role the wife of Justin Ross Harris may have played in the death of their 22-month-old son.
Cooper died last month after being left for eight hours in his dad's sweltering SUV in Atlanta, Georgia.
Investigators have described the mother's behaviour as 'odd' if not suspicious.
They have voiced concerned because she showed little emotion throughout her husband's court hearing last week, where it was revealed he had been sexting with six different women on the day their son had passed away.
Cobb County Police Detective Phil Stoddard said when Mrs Harris was informed by workers at her son's day care facility that Cooper had never been dropped off, she calmly responded: "Ross must have left him in the car. There's no other explanation."
Then, when reunited with her husband at police headquarters after he had been charged with murder, Mrs Harris asked him: "Did you say too much?"
And when she called home on June 18 with the news of Cooper's death, her mother could be overheard on the phone saying: "Why aren't you crying? Why aren't you reacting?"
Det Stoddard said she responded: "I must be in shock."
Police had previously revealed that, like her husband, she had researched children dying in hot vehicles before Cooper's death, telling officers it was her 'worst fear.'
At Cooper's funeral - and with her husband listening from jail on speaker phone - she said she held no anger toward him.
She told mourners: "Ross is and was a wonderful father." She then told her husband of seven years: "I love you and I'm doing this for you."
Some of Mrs Harris' comments during her eulogy at Cooper's funeral have also raised suspicions.
She said: "Some of you might wonder how I'm standing here today and I ask myself the same question.
"I should be crumpled into a pile of tears and snot on the ground. (The Lord) is standing behind me, holding me up."
The 250 mourners gave her two rounds of applause as she said she wouldn't bring her son back, even if she could, adding: "He's in the most peaceful, wonderful place there is."
Cobb County District Attorney Vic Reynolds said the investigation continues and 'much work remains.'
Justin Ross Harris was denied bail on Thursday by a judge who called it a 'possible death penalty case.' He remains in jail on murder and child cruelty charges and prosecutors may seek to use his wife as a witness.