For almost three decades, Mr Cook, 55, would have these words for his children as he left home each morning. "I'm going out to make the streets of America safe for little children, pretty women and old dogs". Glib words for sure, said with tongue half in cheek, but words he earnestly believed in.
One time, Mr Cook and his colleagues failed the city. That was the night of 2 January 1995, when two university students were shot dead as they sat in their car in a lovers' lane area at nearby Lake Juliette. It was only on 4 December 1996, that the Macon police, through gun-sale records, found a suspect. He lived in a trailer near the lake. Name: Andrew Cook.
Andrew was John Cook's son, now 23, and that was the beginning of the agent's nightmare. That day, he received a phone call from Andrew. This is how Mr Cook later recalled asking his son about the night of the murders.
"He was hesitant, and finally said, `Daddy, I can't tell you. You're one of them, a cop'. I said, `Andy, I'm your father. Do you know anything?'" As the conversation proceeded, the boy said that yes, he did. Next, Andrew admitted he had been at the scene. Then, girding himself, John Cook asked the fateful question. "Did you shoot them?" Andrew said that he had.
It was a moment, Mr Cook recalled, that "wrenched my heart out. I felt like the world crashed in on me". But what was to ensue would prove more painful still. Unable to put aside his commitment to the law and to God, even for his own child, he accompanied the boy the next day to turn himself in.
Two weeks ago, Andrew Cook went on trial for two counts of first degree murder. Because of that one conversation - the telephone confession - John Cook found himself in court as the star witness for the prosecution.
The trial, in Macon's courthouse, lasted barely a week. With the words of the father in its ears as well as DNA evidence produced by the prosecution, the jury took two hours to reach its verdict: guilty. Judge Johnnie Caldwell said that the killing of the two young people, Michelle Cartagena, 19, and Grant Hendrickson, 20, was the most senseless he had ever seen.
One more task remained for the distraught father: to plead with the jury in the sentencing hearing the next day not to spare his son from execution. Moving the court to tears and crying himself, he said: "I was busy looking out the front door for evil. But it came in the back door and consumed my son."
He went on: "Yesterday, I sat here and talked to you as the cop, and now I want to talk to you as the father." Asking jurors to accept that there had to be a "kernel of value, of goodness" deep in his son, he concluded: "I knew it would probably be my words that would send him to the electric chair."
He had guessed right. Andrew Cook's confession, given in trust from a son to a father, was too much for the jury to ignore; it showed no hesitation in recommending the death sentence. Judge Caldwell duly obliged and Andrew Cook is now on Georgia's Death Row.
Cook Sr, who resigned from the FBI in February and is now an investigator in the local district attorney's office, has since told the Atlanta Constitution that he survived the trial "not because I am any hero or have special courage. You do what you have to, and I have a strong belief in God.
"God is not finished with the final chapter in any of our lives. I don't know what the final chapter will be for Andy's life, but somehow, somewhere, there will be a purpose."
Does he regret reporting that December telephone call to the police? No. But, he adds, "I probably would not have gone into such detail that I would be the star witness against him."