David Adams vanished in May 1968 on a walk home from a friend's house where he had been playing in his Issaquah, Wash., neighborhood. He was never seen or heard from again, and a body was never discovered.
This one, they say, could be cracked.
"We believe that the Adams' case is solvable," said King County Sheriff's Office spokesman Sgt. John Urquhart said. "We believe this is different."
Investigators from the sheriff's office Cold Case Squad are focusing their attention on a person of interest in the case, a man they think could have been the last person to see Adams the day he vanished.
Urquhart said this man is being investigated because he "does not have a good alibi."
The spokesman declined to identify the person of interest, but told ABCNews.com that he was in his 20s when Adams went missing and was a neighbor of the family's.
The man, who has so far been cooperating with police, is now in his 60s and is a veteran of the Vietnam War, said Urquhart.
"The person of interest lived in the area and had the potential of being the last person who would have seen the boy," said Urquhart. "We don't know that he was, but because of the route the boy took home, we believe he could have been."
One of the hardest parts of retracing the steps of a cold case as old as Adams is notifying the family, said Urquhart.
"We probably get their hopes up and it's difficult," he said of calling Adams' parents, Ann and Don, both of whom are still alive and living in the Kings County area in Washington. "I can't imagine that first phone call after not hearing from detectives in 30 or 40 years."
"It's been 41 years, but they don't even have a body," he said.
While Adams' parents did not immediately respond to messages left by ABCNews.com, the boy's sister, 38-year-old Brooke Vaughan, said that after this many years, some of the family might not even want a body.
"This investigation is good and bad for my family," said Vaughan, who was born four years after her brother disappeared. "It's hard for my parents to go through, to reopen the wounds."
"Obviously we'd like to find out what happened to him, but in some sense my parents feel like he's in a good place and maybe they don't want to know the details of every little incident that happened," said Vaughan.
Vaughan said that her brother's disappearance was not spoken about much when she was growing up. Her other brothers and sisters – four in total – have all reacted differently to the new interest in the case, she said.
Asked whether she believes her brother is still alive, Vaughan said, "No."
In addition to speaking with Adams' neighbor, Urquhart's department has also handed out a deck of playing cards – each emblazoned with a photo of a missing person -- to prisons and jails.
"As time goes on people who might have information or known a suspect tend to come forward," he said. "They're not as afraid of the suspect anymore or they might be on their death bed and what to get it off their chests."
"We are hopeful," said Vaughan. "We always are."