(CNN) -- It's been 31 years since Stanley Guidroz first reported his 3-year-old son missing in Tacoma, Washington. 

Investigators, after reopening the cold case, now say the boy, Wallace Guidroz, was never missing at all and that his father had killed him. 

Guidroz, 57, has been charged with first-degree manslaughter after he confessed to the crime, said Pierce County Prosecutor Mark Lindquist on Tuesday. 

"This is another success for the Cold Case Unit," said Lindquist. "Justice matters, no matter how long it takes." 

Guidroz first reported his son missing in January 1983, telling Tacoma police that his son had gone missing in a neighborhood park after they had gone fishing together. Guidroz said he believed that a family they had met in the park had kidnapped him, prosecutors said. 

Police were unable to locate a family that fit the description Guidroz gave them, and they were unable to connect Guidroz to the boy's disappearance. 

For years, the mystery of what happened to Wallace remained unsolved. 

Guidroz left Washington state in 1984 and did not return, according to a press release from the Pierce County Prosecutor's Office. 

In 2011, Cold Case Unit Detective Gene Miller reopened the case. Miller reviewed old documents from the initial investigation and saw that Guidroz's stories never seemed to add up. 

There were "several inconsistencies in Guidroz's account of the events from the day Wallace went missing," the release said. 

Guidroz resurfaced in Louisiana in March 2011 after reports that he had been charged in the killing of his wife, who is not the mother of Wallace. 
when was the case reopened?
In 2011, Cold Case Unit Detective Gene Miller reopened the case