Hot car death trial: Witness breaks down in tears as he recalls attempts to save 22-month old Cooper Harris

James Hawkins, a lighting installer, testified at the trial of the child's father

All efforts to revive the toddler were in vain (CBS)

A witness broke down as he described his frantic efforts to revive a toddler who died after being left in a hot car.

Cooper Harris died in June 2014 after being left for seven hours in his father’s vehicle outside his workplace near Atlanta. Lawyers for the boy’s father, Justin Harris, have argued that it was a tragic accident and that he simply forget to drop off the boy at day care.

But prosecutors have alleged that Mr Harris left the child to die on purpose as he sought to escape the responsibilities of family life and focus on sexual liaisons with prostitutes and young women he met online.

CBS said that at a trial in Georgia, where Mr Harris has denied a charge of murder, a man who tried to save the 22-month-old boy broke down and wept. James Hawkins, a lighting installer, testified that he had rushed over to help and tried to administer CPR.

“It was [like] blowing into a busted bag,” he said, wiping away tears. “He was gone. He was dead.”

He said he left the scene without talking to police, distraught because he had lost his own daughter a year earlier. “I was really upset,” he said on Tuesday.

The day before, prosectors had said there was no doubt that Mr Harris had intended to kill his child. But the defence team attempted to rebut this, while admitting the death of the toddler was the father’s fault.

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“Ross Harris is responsible for his child’s death. It’s his fault, no doubt about it,” Maddox Kilgore told the jury in his opening statement.

“What you’re going to see here at this trial is that being responsible is not the same thing as being a criminal.”

Before Mr Hawkins took the stand, a police officer testified Mr Harris was initially combative with officers and later complained that he was hot in the back of a patrol car.

Cobb County police officer Jacquelyn Piper said Mr Harris was on the phone when she arrived at the scene and initially refused to get off, using an expletive when telling another officer to shut up.

Justin Harris has denied a charge of murder (AP)

“He started a monotoned yelling that seemed really forced,” Ms Piper testified, according to the Atlanta Journal-Constitution. “I found it kind of unusual.”

Ms Piper said Mr Harris also insisted he had to call his wife because she was going to go to Cooper’s day care and he would not be there and later made casual conversation while she was taking him to police headquarters.

Once he was in a cruiser, Ms Piper said Mr Harris complained about how hot it was in the car. He’s seen on police video sitting in the cruiser while Ms Piper tells him the “air conditioning is all the way up.”

Mr Harris faces life in prison if he is convicted of murder. Prosecutors decided not to seek the death penalty. The case drew national attention and was so closely followed in Mr Harris’ suburban home of Cobb County that a judge moved the trial 275 miles to Brunswick, on the Georgia coast.

Sixteen total jurors - eight men and eight women - were seated Monday morning to hear the case. Four of them will serve as alternate jurors, who will have a final say in the trial only if one or more of the 12 main jurors are dismissed.