Michael Devlin, 41, a pizza parlor manager, is accused of kidnapping two young boys and holding one of them for four years.
Shawn Hornbeck, 15, had several chances to escape but experts say he was probably too fearful to run from his captor.
Ben Ownby, 13, was snatched by Michael Devlin on his way home from school days ago, police said.
Two police officers who blew open a huge kidnapping case frequented the pizzeria where the alleged abductor worked.
While working another case at Michael Devlin's apartment complex last week, Officers Gary Wagster and Chris Nelson just happened to notice that Devlin's truck matched the description of a vehicle believed to be involved in the kidnapping of 13-year-old William Ben Ownby.
William was snatched Monday on his way home from school.
When Devlin appeared to take his trash out, the officers approached him and asked him questions.
At first friendly, Devlin's "whole demeanor changed," Nelson said.
"It went from a casual conversation, 180 degrees from that," he told CNN.
Wagster and Nelson recognized the 41-year-old manager of Imo's in Kirkwood, a St. Louis, Missouri suburb.
Nelson even said he remembered Devlin from having once arrested him for a traffic violation.
"So I had kind of a rapport with him," Nelson said.
An 'uneasy feeling' leads to a 'miracle'
Though the officers would not discuss details of the conversation, Wagster said they left the apartment complex with "an uneasy feeling," and decided to alert the FBI.
"For one of us to have that (feeling), that's one thing," Wagster said. "But for both of us to have it, as much as we've worked together, we knew that there was something wrong."
Investigators returned to Devlin's apartment on Friday to find not only William, but 15-year-old Shawn Hornbeck who had been reported missing in October 2002.
Devlin was arrested and has been charged with one count of first-degree kidnapping. He was being held on $1 million bail, and prosecutors say more charges are likely.
The arrest stunned his friends and neighbors.
Bill Romer, Devlin's landlord, said the suspect had identified Shawn as his son and listed him as a second occupant in the unit.
"I'm trying to figure out what exactly I would do differently," Romer told CNN. "You know, if I had run a background check on Devlin, I would have gotten a traffic violation come up."
And Imo's owner Mike Prosperi said Devlin's discussion of his personal life usually involved his mother or brother -- "never any mention of children."
"He's worked for me for 25 years," Prosperi told reporters. "There's not too many people who stay in the same job for 25 years. He was my manager. He counted my money, and you just don't do that with someone you don't trust."
But Prosperi said he became suspicious of Devlin when he thought about his employee's white truck and the sick day he took the day William disappeared.
He told St. Louis television station KSDK that he reported his concern to a friend in the Kirkwood police department. His friend said investigators were getting ready to look into the tip when Wagster and Nelson spotted the truck themselves, according to Prosperi.
He added, "I just can't understand how all that time -- nobody knew."
Police are looking into that same question, particularly in Shawn's case.
Now 15, Shawn was reportedly last seen more than four years ago in his hometown of Richwoods, Missouri, about 50 miles south of Kirkwood.
Neighbors said the teen and Devlin appeared to be a normal father and son.
Dr. Jeffrey Lieberman, chairman of the psychiatry department at Columbia University College of Physicians and Surgeons in New York, called Shawn's case "troubling and puzzling."
"He obviously was not being monitored by Devlin 24-7, and he was out on his own, leading apparently normal activities and even seeming to enjoy himself," Lieberman said. "Why didn't he try to escape? Why didn't he reveal his circumstances?"
Lieberman said Shawn may have fallen victim to "Stockholm Syndrome," in which hostages or kidnapping victims sometimes befriend their captors.
The name was coined by a criminologist involved in a 1973 case in Sweden in which bank robbery hostages became emotionally attached, even defensive, of the robbers.
The most famous case of Stockholm Syndrome concerned newspaper heiress Patty Hearst who, in the 1970s, helped her captors rob banks.
Lieberman called it "a coping mechanism" for people in life-threatening situations.
"The captor can also encourage this by being nice to the person, by giving them gifts, by making them think they will be treated well, and even by making them think this will be a pleasant and enjoyable and better experience than they previously had," he said.
William had last been reported seen on Monday getting off a school bus in Franklin County, west of St. Louis.
His family said Saturday the boy told them he saw signs and flyers posted for him.
Roland Corvington, the special agent in charge of the FBI's St. Louis office, said when police entered Devlin's apartment, William looked up and asked, "Are you going to take me home?"
Shawn identified himself to police when asked, Corvington said.
His stepfather, Craig Akers, said Saturday that his family would talk with him about the abduction "when Shawn's ready to discuss it."
Authorities have released no details of what the boys went through.
Franklin County Sheriff Gary Toelke said investigators have not yet questioned the boys.
Both were spending a quiet day at home with family on Sunday.