A doctor who asked not to be named said Baiji hospital received eight bodies after the incident, including a 60-year-old man.
Reuters photographs showed relatives of the dead standing beside their corpses covered by white sheets outside a mosque in Baiji, 180km north of Baghdad.
Lieutenant-Colonel Maura Gillen, a US military spokeswoman, said the military was following up the incident, in which "suspicious activity" had been noted and warnings to stop the car were ignored.
The incident comes at a sensitive time.
Two Iraqi journalists were killed in separate incidents in Baghdad and the province of Diyala on Wednesday, police and their employers said on Thursday.
Wissa Ali Ouda, a cameraman for private television station Afaq, was killed by a sniper as he was heading home in the al-Obeidi district of northeastern Baghdad, Khazaal Ghazi, his editor-in-chief, said.
Tariq Maher, a correspondent for Afaq said the 30-year-old cameraman was apparently caught in the crossfire in eastern Baghdad as he was on his way home.
The body of another journalist, Haidar Hashem al-Husseini, was found in the town of Baquba northeast of Baghdad on Wednesday, two days after he was kidnapped, a local police official said.
At least 127 journalists and 50 media workers have been killed in Iraq since the 2003 US-led invasion, according to the New York-based Committee to Protect Journalists. That figure does not include Ouda and al-Husseini's deaths.