He frequently acted as a spokesman for witnesses in police or court cases, and had been acting as an adviser to Nabil B, a former gang member testifying in the case against alleged drug lord Ridouan Taghi.
Mr de Vries had previously faced danger because of his work and needed police protection after receiving threats in connection with his coverage of criminal cases.
In 2019, police told him he was on the hit list of the Netherlands’ most wanted criminal.
EU Commission President Ursula von der Leyen tweeted her condolences, with the message:
“Investigative journalists are vital to our democracies. We must do everything we can to protect them.”
The two men held on suspicion of the murder are a 21-year-old from Rotterdam and a 35-year-old Pole. Police have not described them in any detail.