De Vries was shot in a busy Amsterdam street on July 6, 2021, and died nine days later, raising concerns about the ability of the criminal underworld to eliminate a prominent public figure who was considered a threat.
Prosecutors had demanded life sentences for the Dutchman who shot De Vries and for two men from Poland who had been charged with planning the murder and trying to help the assassin escape.
Three other men were sentenced to up to 14 years for aiding the murder, which the court said had "gravely shocked" Dutch society.
None of the suspects had made any statement during the trial.
Peter R De Vries was acting as an adviser to a suspect-turned-state witness when he was shot. (AP PHOTO)
De Vries, 64, was well-known for his television programs in which he often worked with victims' families and tirelessly pursued unsolved cases.
He had received threats from the criminal underworld in connection with his work.
In 2008 he won an international Emmy Award in the current affairs category for his work investigating the 2005 disappearance of US teenager Natalee Holloway in Aruba.
At the time of his death, De Vries was acting as an adviser to a suspect-turned-state witness in the trial against Ridouan Taghi, who was sentenced to life in prison for murder and drug trafficking earlier this year.
The state witness's lawyer, Derk Wiersum, was gunned down in front of his home in Amsterdam in 2019.
Taghi took the unusual step in 2019 of making a public statement denying reports that he had threatened to have De Vries killed.
Prosecutors in the De Vries case said they were convinced Taghi had ordered the Polish men to organise the murder, but Taghi was not part of the trial.
The court said it had therefore not been able to establish a link between the De Vries' murder and his role as adviser to the state witness.
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