A Dutchman who tried to drive his car into a bus carrying Queen Beatrix at a royal parade died from his injuries today.
A military policeman hit by the vehicle as it ploughed through a crowd also died, taking the toll among onlookers to six.
Mourners laid flowers at the monument in the city of Apeldoorn which the man's car hit after narrowly missing the open-top bus carrying Beatrix and her family yesterday.
The motive of the 38-year-old driver, identified by local media as Karst Tates, was unclear. No weapons or explosives were found when police searched his house and there were no signs that others were involved in planning the attack.
Authorities plan an investigation into the security measures surrounding the parade, the Justice and Interior ministries said in a statement today.
The public prosecutor said Tates died overnight. He had been charged with an attempted attack on the royal family and had told police he had deliberately targeted the royals.
The prosecutor said Tates came from Huissen, a small town about 25 miles south of Apeldoorn.
Newspapers cited his neighbours as saying he was a withdrawn man who lived alone. He had lost his job at a security firm and was due to move house, the AD paper reported.
The military policeman died from his injuries today, the Defence Ministry said. Eight people are still in hospital, with one woman in a critical condition following the attack on the Queen's Day national holiday.
Flags flew at half-mast today and people in Apeldoorn signed a book of condolences. Many wondered if the holiday could be celebrated again.
"Sunny orange is coloured deep black," read one newspaper headline, referring to the Dutch national and royal colour. Another asked "Is this the end of a tradition?" next to a photo of victims thrown to the ground.
"Queen's Day is the one outstanding day on which a deeply divided Netherlands celebrates the fact that it also knows unity," said Rijk Timmer in Dutch daily Het Financieele Dagblad. "An attack on this day hits the foundations of our society."
Queen Beatrix was accompanied by Princess Maxima, wife of heir Willem-Alexander, and her other children in the open-top bus which was heading to a palace in Apeldoorn, about 90 km (56 miles) east of Amsterdam, as part of the annual festivities.
Following the attack, events linked to war remembrance on Monday and Tuesday in Amsterdam were under review, though the royal family is still due to make public appearances.