Karst Tates killed the bystanders and himself when he ploughed through layers of spectators waiting to catch a glimpse of the royal family on 30 April, the Queen's Day national holiday.
Prosecutors yesterday said they still do not know for sure Tates' motive for the attack in the central city of Apeldoorn, which played out live on national television to a horrified public.
"We have no final answer to the ultimate question, why? That answer had to come from Karst himself," said the country's top prosecutor, Bart Nieuwenhuizen.
Police said the 38-year-old man confessed as he lay slumped and bleeding in his car that he intended to hit the royal family. He went into a coma shortly afterward and died of his injuries the following day.
Among his final words, Tates called Crown Prince Willem-Alexander a "fascist … a racist," said Tom Driessen of the national investigation bureau.
But Driessen added that Tates had not based on any "ideological" cause.