So when they discovered that same father had been robbing small-town banks, the three sons put his tutelage to the test: They turned him in.
Now William Alfred "Al" Ginglen, a 64-year-old grandfather of seven, could spend the rest of his life in prison. He pleaded guilty in July to seven counts of armed bank robbery and two counts of carrying and using a firearm during a crime of violence; sentencing is scheduled Thursday in federal court in Springfield.
His sons say they have no regrets.
"He turned to crime, and we had an opportunity to stop it," said Clay Ginglen, 36, a music teacher in his hometown of about 2,600 people. "He was robbing banks with a gun. He could have easily hurt anyone: a bank teller, a policeman. He could have been hurt as well."
Ginglen's double life, which authorities allege included a girlfriend, drugs and prostitutes, started to unravel in August 2004, when one of his sons, Peoria police Officer Jared Ginglen, looked at surveillance videos posted on a law enforcement Web site and recognized his dad behind sunglasses, a dust mask and driver's cap.
He called brother Garrett Ginglen, 41, a Caterpillar Inc. engineer, who says he broke into a sweat and threw up in his office trashcan when he called up the photos.
"I felt like if I could I would get up and run as fast and far as I could," he said. "Just trying to get away from it and pretend like it didn't happen."
The three brothers quickly gathered at the Lewistown firehouse where Garrett and Clay Ginglen volunteer and decided to confront their father.
He wasn't home, but the sons found clothes that matched those worn by the robber. They called police, who arrested Al Ginglen the next morning outside the home of a woman authorities say he had been secretly seeing since the 1990s.
Along with a gun used in at least two of the robberies, the investigation turned up a journal Ginglen kept that prosecutors says details the robberies and the double life they bankrolled.
Ginglen wrote that he needed money to support his girlfriend and her daughter, and to pay for a $400 to $900 a week crack cocaine habit and hotel rooms where he romped with prostitutes, prosecutors said.
His sons say the family was oblivious to the nine-month robbery spree, which netted nearly $60,000 from central Illinois banks, and their dad's secret life.
"There's a lot of things we're upset about that weren't illegal," Clay Ginglen said. "Lying's not a crime, and lying was the biggest thing."
Al Ginglen told the Chicago Tribune the journal was a fictionalized outline for a book he planned to write. He declined an interview request from The Associated Press.
His sons, who say they have read only parts of the journal, rejected the explanation.
"I think it was a way that someone who was living a double life would try to keep track of his stories, to not slip up and get caught," Clay Ginglen said.
In hindsight, Ginglen's sons now recognize clues that their father's life had been unraveling.
After being laid off for about a year when Maytag began shuttering its Galesburg refrigerator plant in 2002, he told his family he had landed a job collecting receipts from video games in bars and restaurants across central Illinois. He was away from home three to four days a week and called his sons frequently for money, they said.
"Looking back now, he was not behaving like he used to," Garrett Ginglen said.
His attorney, Ron Hamm, did not return a call for comment, but has said Ginglen was a devoted family man with a history of community service and no criminal record before the robberies began in 2003. Prosecutors declined comment.
Whatever the sentence, Ginglen's sons hope their father someday realizes it was the lessons he taught them that landed him in jail and may have saved his life.
"We knew he could be mad. It wasn't like we didn't mow the lawn when we were supposed to," Garrett Ginglen said. "But we also hoped that since he taught us all of this and raised us to be good, maybe someday the light bulb will come on."