The dead were among nine French expatriates who were ambushed in a desert area some 50km north of Medina.
Two of them died on the spot, Hajjaj said. A third victim was driven to a nearby medical centre by his wife in defiance of a ban on women driving in conservative Saudi Arabia but was pronounced dead on arrival.
On Monday, Major-General Mansour al-Turki, a spokesman for the interior ministry, said the group of French expatriates were resting on the side of a road when shots were fired at their car.
|A French teenage survivor leaves the King|
Fahed hospital in Medina [AFP]
"They are all residents who work in Riyadh and who had gone on a desert trip," he said.
The Frenchmen were returning from Madain Saleh, a popular destination for Western expatriates living in the kingdom.
A security source said the attackers had singled out the men.
Al-Turki said: "Undoubtedly this is a criminal act, but it's too early to determine their motives and whether it is terrorism."
In Paris, Jacques Chirac, the French president, called for action to find the attackers and ensure foreigners' safety.
"He strongly condemns this hateful act," a statement said, calling on authorities to "track down, try and punish those guilty of the act and guarantee the safety of our compatriots".
Grand Mufti Sheikh Abdulaziz Al al-Sheikh, the most senior government-appointed cleric, said Saudis should condemn the attack.
He said on state television: "All citizens have a responsibility to warn against this [kind of] crime and condemn it."
Philippe Douste-Blazy, the French foreign minister, said the French ambassador to Saudi Arabia and consul-general were planning to visit the scene of the attack and discuss the circumstances of the incident with Saudi authorities.
"I condemn this horrible act. I express my profound sympathy to the family and friends of the victims.
"French authorities are mobilised along with the Saudi authorities, so that light be shed and that those responsible be detained and punished."
The attack was the first on Western expatriates since a Briton was wounded in a knife attack in November in the eastern industrial city of Jubail.