The armed men ordered the passengers out of the vehicle and lined them up before shooting them in daylight on Sunday afternoon, police on Monday quoted witnesses as saying.
They did not specify whether the victims were civilians or off-duty security personnel but said they were believed to be from the neighbouring town of Saqlawiyah, in the Sunni Triangle.
Pools of blood covered the scene of the attack on the desert road, next to empty cases of pistol and AK-47 machine gun bullets, suggesting that the victims were shot from close range, an AFP correspondent reported.
The road was blocked for nearly 10 hours until tribesmen from the neighbouring Sunni town of Ramadi removed the bodies, police added.
Security forces accused
The main Sunni Arab party in Iraq accused Shia-led security forces on Monday of involvement in the execution of 37 Iraqis who were found dumped in a river last week.
"The full responsibility falls upon the current government, especially the Interior Ministry, which, if not directly involved in this crime, is providing cover for the culprits," the Islamic Iraqi Party said in a statement.
"Otherwise, how can a large group of individuals and vehicles move at such a late hour in the night, breaching curfew without having an official permit."
US forces arrested suspected
fighters in raids in Mosul
Iraqi police on Thursday said they found the bodies of 37 men, each executed with a bullet to the head and dumped in a stream south of Baghdad.
The decomposing bodies, all wearing civilian clothes, were thought to have been dead about five days and had their hands bound or handcuffed.
US forces detained 17 suspected fighters in a series of raids in and around Iraq's main northern city of Mosul, the US military said on Monday.
Twelve were detained in operations in Mosul, four in the town of Tal Afar farther west and one in an unspecified part of the surrounding Nineveh province.
Weapons and bomb-making equipment were recovered, the military said.
Mosul has long been a stronghold of Sunni Arab fighters, while US commanders say Tal Afar has become a major staging point for foreign fighters infiltrating Iraq from Syria and other Arab countries.