"This has really been a tragedy for two families, a tragedy for a class, as well as all of South Dakota, to some degree," state Attorney General Marty Jackley said this week.
The questions began on the evening of May 19, 1971. After visiting Miller's ailing grandmother at a hospital, the girls met up with some boys at a church parking lot and started to follow them to the quarry. Miller drove the beige 1960 Studebaker Lark that had belonged to her grandmother, who died shortly after she disappeared. Jackson was in the passenger's seat.
The boys missed a turn and accidentally drove past the party. When they turned around, they told authorities, they no longer saw the Studebaker's headlights. They figured the girls had simply lost the nerve to attend the party.
The celebration went on, but the girls and the Studebaker would not be seen again until 2013.
In the following weeks, volunteers and law-enforcement officers searched the gravel pit, surrounding roads and even the nearby Missouri River. But their efforts yielded nothing. The mystery persisted, year after year, for more than four decades, tormenting the girls' families, baffling investigators and inviting false allegations against a sex offender.
Every time the girls' classmates reunited, the disappearance came up.
"We were always curious," said Dwight Iverson, a classmate who now runs a restaurant and convenience store in nearby Vermillion. "We would talk about it. That car has to be somewhere on this planet, and it's never been found."
At one point, the state's cold case unit reopened the investigation after a prison snitch implicated a fellow classmate who had lived nearby and was behind bars for raping a woman. Authorities concluded he made the story up.
Then in September, flooding followed by a drought brought the car into view. It was upside-down in Brule Creek next to the gravel pit where the girls had been headed. A fisherman spotted two tires sticking above the water.
On Tuesday, authorities held a news conference to confirm what townspeople had suspected since the car re-emerged: The remains inside were those of Miller and Jackson.
The evidence seemed frozen in time. A picture of Mount Rushmore on the white license plate was clearly visible, as were the green registration numbers. A watch still had its strap and clearly showed the time it stopped, 10:20.
Miller's purse was intact, containing her driver's license, coins, a couple of letters from friends and other items, all in relatively good shape for being submerged for so long. Those belongings and DNA were used to identify the remains.
There's no evidence the teens had been drinking. And mechanical tests on the car did not suggest any foul play. The headlight switch on the dashboard was on. The car was in high gear, and both girls were found in the front seat. Those factors point to an accident, Jackley said.
Investigators would not speculate on exactly what happened, though one of the tires was damaged and the tread was thin, Jackley added.
Ray Hofman, who knew the Miller family and searched for the teens during his career with the Vermillion Police Department, said the two probably lost track of the narrow, dusty road and accidentally drove into the creek.