"I think that it has become clear that this happened with premeditation," Prime Minister Jan Peter Balkenende said.
Prosecutors said the driver, badly injured and still in his crumpled car, acknowledged targeting the queen and her family. "The man said that his action was aimed at the royal family," said prosecutor Ludo Goossens.
The driver, whose name was not released, "is formally suspected of ... an attack on members of the royal house and manslaughter or murder," Goossens said, and he could face life in prison.
The motive for the attack was unclear. Dutch media, citing neighbors, said the assailant recently was fired from his job and was to be evicted from his home. Police identified him as a 38-year-old Dutch man with no history of mental illness or police record, but they would not release his name.
Officials in Apeldoorn said the driver had a map of the queen's route.
Celebrations were canceled for Queen's Day, the national holiday that was to draw millions of people to street dances, picnics and outdoor parties under sunny skies around the country. Flags were lowered to half staff. The Dutch Embassy in Washington canceled a scheduled reception.
A shaken Queen Beatrix extended her sympathies to the victims in a brief nationally televised address. "What began as a great day has ended in a terrible tragedy that has shocked us all deeply," she said.
The driver apparently acted alone and was not linked to any terrorist or ideological group. No explosives were found in his car or in his home, said Goossens.
"From initial contact with police before the suspect was removed from the car ... we have reason to believe it was a deliberate action," Goossens told reporters.
The driver apparently tried to intercept the bus as it turned a corner to a road leading to the gates of the Het Loo palace a few hundred meters (yards) from the intersection in this eastern Dutch town.
Though the sequence of events was still murky, he apparently crashed his small black car through two sets of police barriers, smashing his windscreen and damaging the front of the vehicle even before slamming into the monument.
The final few seconds were captured on video and film by news teams following the royal family in a press bus.
Reporters saw people thrown high in the air from the impact or tumbling down the street, their broken limbs askew. First aid crews and police officers ran to the victims and applied revival techniques.
The driver, bleeding from the head and nose, was slumped against the seat when police lifted him out and put him into an ambulance.
Earlier, Apeldoorn Mayor Fred de Graaf said eight of the 13 injured were in serious condition, with two men and two women killed. Later, a third man died of his injuries, said Apeldoorn municipality spokesman Toon Schuiling. Two teenagers and a 9-year-old girl were among the severely injured.
"We are speechless that something so terrible could have happened," the queen said in a rare televised appearance. "My family, and I think everybody in the country, sympathize with the victims, their families and friends and all who have been hit so hard by this accident."
Dutch television footage showed Crown Prince Willem-Alexander and his wife, Princess Maxima, standing at their seats in the bus's high open platform and watching in astonishment. Maxima held her hand over her mouth in apparent horror.
The bus was not hit and no one in the queen's entourage was injured.
A policeman narrowly escaped injury when he jumped off his bicycle to avoid being hit.
Shortly after the crash, investigators and a sniffer dog examined the car for explosives, then sliced off the roof of the car for a closer inspection.
Journalist Peter von de Vorst told RTL television that the incident was like watching a horrible movie.
"It was a really nice day. Then you hear a bang. Everyone looks up and you see people indeed flying through the air. This must be a joke or a strange prank. Then suddenly panic, and you realize that something really terrible has happened," he said.
Hundreds of thousands of people flocked to the main Dutch cities on Wednesday night and Thursday to celebrate the national holiday, originally intended to celebrate the birthday of Beatrix's mother, Queen Juliana.
The royal family normally spends the day in a small Dutch community.
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