The man, a 62-year-old Clinton Township resident, has not been charged and authorities were awaiting toxicology results from an autopsy performed on the body of the 31-year-old woman who died.
Police said the man and his 92-year-old mother spend their winters in Arizona and were returning to Michigan with a 31-year-old woman with whom the man said he had had a romantic relationship.
When officers arrived Tuesday at his son's home in Warren, just north of Detroit, the man was weeping on the curb and his mother was in her wheelchair in the back of the van. The dead woman's corpse was in the front passenger seat wearing a seat belt and sunglasses.
"She obviously had been dead for at least 24 hours in screeching heat," said Warren Police Commissioner Jere Green.
Police did not release the names of the man or the dead woman.
Their 1,700-mile journey began Sunday in the Phoenix area after the woman checked herself out of a mental health facility there. At some point the woman, who had a history of substance abuse problems, may have taken oxycodone, Green said.
"They stopped in Flagstaff and she went in to use the bathroom," Green said. "We're guessing she might have overdosed."
Green said the driver later tried to wake her but discovered her body was cold and presumed that she had died. He did an Internet search on his cellphone and later told police he read something about having 48 hours to take a corpse to a medical examiner or morgue.
As their macabre journey continued, someone at the mental health facility in Arizona called the woman's cellphone to check on her.
"It's a courtesy call," Green said. "He answers and said, 'She can't talk. ... She's dead.'"
The caller told him to immediately contact police. The driver didn't and later told investigators he was afraid police would arrest them and seize his van.
The facility filed a missing persons report and police contacted the driver, who met them Tuesday. The man has not been arrested and the case is under investigation.
The man gave police explanations for his actions, "which made sense to him," Green said. But "was he committing a crime or was it stupidity?"