According to the Belga news agency, the Belgian federal prosecution is investigating the incident, but the suspects have not been officially announced yet.
The investigators believe that the group, allegedly involved in the cyberattack on the OPCW and the 2016 hack of the US Democratic Party emails, may also be responsible for stealing the diplomatic report, the media claimed.
The group is reportedly suspected to be part of the APT28 team, also known as Fancy Bears.
On Thursday, Dutch Defense Minister Ank Bijleveld announced that four Russian citizens had been expelled from the Netherlands on April 13 on suspicion of an attempted cyberattack on the OPCW, adding that the suspects had diplomatic passports.
The Russian Foreign Ministry, in turn, has said that the anti-Russian "spy mania campaign" unleashed in the Netherlands seriously harms bilateral relations with Moscow. The ministry added that Moscow has not and will not launch such attacks, as it has no reason to do so.