It is a behavior that's hard to fathom -- parents driving drunk with their children in the car. Yet studies show that two thirds of children who die in drunken driving accidents are killed by the very people in charge of caring for them.
"Criminal behavior that puts our children at risk" is inexcusable," said Carl McDonald, a former lieutenant with the Wyoming State Highway Patrol. McDonald said he considers combating drunk driving his life's work.
"As a trooper, I was very aggressive on drunk driving, and I was proud of my ability to pick up on that," McDonald said in a recent interview in New York. "I would walk up to a car and from eight feet away on initial contact with the driver of a car, I can tell you whether or not that driver was sober or intoxicated."
But there would be one drunken driver whom McDonald did not suspect, and who changed his life forever.
"I was living my life around a woman who was intoxicated often and [I] didn't know it," said McDonald.
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The woman McDonald says he didn't suspect was his wife, Susan, a dispatcher with the sheriff's department, and the mother of their only child, Carlie.
Susan McDonald says her drinking increased over the years and, at her worst, she was a raging alcoholic.
"I'm sure I was drinking at least a fifth of whiskey a day," she told "20/20." "And I was driving."
Because of her drinking, Susan lost her job, her three-year marriage to Carl ended and, in the divorce, Carl sought and won primary custody of Carlie.
For two-and-a-half years, Susan would see Carlie on a regular basis, but then Susan's drinking escalated and, for several months, she couldn't bring herself to see her daughter, as difficult as that was for her, she said.
"Carlie was born when I was in my mid-thirties, so I was more comfortable being a mom," said Susan. "She was just a good little person."
It's a sentiment that Carl shares.
"She was a social creature," he said. "When she got around other children she would jump right in, and she had a tendency to take charge of things."
Carl and Carlie were very close, spending lots of father-daughter time -- camping, fishing. Carlie would even help Carl at work.
"We did a number of programs with other children together where we demonstrated how seat belts work," said Carl. "She wouldn't let you start a car without fastening your seat belt."
Carl was spending more time with Carlie, as Susan spent less. Carl had his suspicions that Susan's drinking was why she wasn't seeing Carlie, he said. Then, after Christmas in 1997, Susan called, wanting to spend New Year's with Carlie.
"I was worried, as always, about my daughter's safety, but I was also in this position with court orders that said I would not interfere with custody, visitation, lifestyle issues," said Carl.
So Susan picked up her 5-year-old, and they headed to Susan's home. On New Year's Day, Carl called to wish Carlie a happy 1998.
"As usual, I told her I loved her," he said. "I knew that she would be coming to my house the next day, and I told her I couldn't wait to see her again."
Later that day, after Susan had been drinking, she took Carlie out to visit her older daughter. Physically unable to get the child into her car seat, Susan put her in the front, where Carlie dutifully buckled up.