Last month, Victor Gevers said that he had guessed the US president's Twitter password was 'MAGA2020!', and had been able to use this to access his account.
The White House denied this had happened and Twitter also said there was no evidence that such a breach had occurred.
However, the BBC reports Gevers has now come up with more information to support his claims.
A spokesperson for the Dutch Public Prosecution Service told De Volkskrant newspaper: "We are currently investigating whether something criminal has happened."
Gevers allegedly took screenshots of the hack which he shared with Dutch newspaper de Volkskrant.
He told the publication: "I expected to be blocked after four failed attempts. Or at least would be asked to provide additional information."
But with such huge power in his hands, Gevers said he did nothing more than email Trump - using an account of the President's he had previously acquired - advising him to enable two-step verification and suggesting he use a stronger password, such as '!IWillMakeAmericaGreatAgain2020!'.
Twitter rejected the alleged hack, stating there is no evidence to support Gevers' claim.
A spokesperson said in a statement at the time: "We've seen no evidence to corroborate this claim, including from the article published in the Netherlands today.
"We proactively implemented account security measures for a designated group of high-profile, election-related Twitter accounts in the United States, including federal branches of government."
If Gevers isn't fibbing, then this will actually be the second time he has had access to Trump's account.
He and two others successfully managed to log in prior to the election in 2016, when the password was 'yourefired', according to Vrij Nederland.
Speaking after the incident, Gevers said he noticed that two-step verification has been activated on Trump's account. He also said the American Secret Service got in touch to ask him a number of questions and also thank him for making them aware of the security leak.
Gevers said: "Why is it possible for someone from a different time zone to log into such an important account? Why doesn't Twitter demand better passwords?
"If I can access his account, then foreign nations can do so as well, right? Why aren't the persons who are supposed to protect the President informed when someone reports that his account is unsafe?"
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