It appears they have enough evidence (because they got an arrest warrant from a judge) that he is part of a sovereign citizen group, and we all know how violent some of them can be just from USA news stories.
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"A day later, the police published a report on the website politie.nl linking Van Kessel to “a criminal network.” According to a press release issued by the Public Prosecution Service, eight people were arrested that morning for “adhering to anti-institutional ideology and possibly intending to use violence.” One of them was quickly released, while the other seven were held in restricted custody for two weeks, meaning the suspects were not allowed any contact with the outside world.
Van Kessel – as was announced last week – was being held in a cell in Vught.In recent weeks, several stories have appeared in the mainstream media about a network of so-called sovereigns. These “anti-institutionalists” may have been planning something related to the NATO summit. Weapons and explosives may have been found, but any hard evidence or substantiation remains lacking to this day.
The suggestion that Van Kessel is also part of a dangerous criminal group has been raised. The charges have since been partially withdrawn. The AD newspaper reported last week that the Public Prosecution Service has been unable to substantiate a plan to disrupt the NATO summit. “We have investigated whether there is an imminent threat. This has not been proven.”
On Thursday, June 26, the Public Prosecution Service released more news after a long silence. One suspect has been released, but “six suspects in the investigation into a criminal network, in which a large proportion of the arrested suspects espouse anti-institutional ideology and may have the intention to use violence, will remain in custody for an additional 90 days,” the Public Prosecution Service announced.
Van Kessel is one of those suspects who will remain in custody for another 90 days. The Public Prosecution Service states that it needs more time for the investigation and that “given the state of the investigation, it is not possible to respond substantively to questions about the progress, suspicions, and findings,” according to the Public Prosecution Service."
Lawyer Suing Gates & Bourla for Covid VAX Injuries Arrested and Imprisoned in Netherlands - LewRockwell
The police have every right to investigate what they deem as anti-institutional ideology with a potential or realized intention to use violence - that's also known as domestic terrorism.