Their perseverance has led to the indictment and arrest of David L. Lykken, 52, who is already serving a 227 year sentence in the South Dakota Penitentiary for kidnapping and rape.
Lykken was indicted Friday, June 29, on two counts of premeditated murder, two counts of felony murder and two counts of murder.
He was arrested Monday, July 2, at the penitentiary where he has been incarcerated since 1990.
The indictment charges that Lykken murdered Cheryl Miller and Pamella Jackson on or about May 29, 1971.
His arraignment is set for 9 a.m. Monday, July 9 at the Union County Courthouse in Elk Point.
The Union County grand jury charged Lykken with two counts of killing Miller and Jackson "with a premeditated design."
Two other counts charge Lykken with rape and murder of Miller and kidnappingand murder of Jackson.
The final two counts contained in the indictment charge the 52-year-old man with murdering Miller and Jackson by "evincing a depraved mind, regardless of human life, although without any premeditated design to effect the deaths" of the two girls.
A number of leads have turned up over the years, ranging from reports of unidentified female bodies found in other states, to claims by captured serial killers that some of their victims were from South Dakota.
None of the claims brought officials closer to solving the case.
In 2004, the state DCI's newly formed Cold Case Unit decided to concentrate on the mystery of the two missing girls.
On Aug. 24, 2004, authorities from the Clay and Union sheriff's offices, the Vermillion Police Department, and the South Dakota Department of Criminal Investigation began the execution of a search warrant at the farm of David Lykken's brother, Kerwyn Lykken, rural Alcester.
David Lykken was 17 and residing at the farm at the time of the girls' disappearance.
According to a 1991 investigative report in the Plain Talk, Miller and Jackson, both 17 and both juniors at Vermillion High School, had arranged to get together the evening of Saturday, May 29.
Jackson told her parents that they were going to visit Miller's grandmother at Dakota Hospital and then "go driving."
Miller told her half-sister, Rita, that after visiting the hospital, she and Jackson were going roller-skating at River Sioux, east of Vermillion, and were then going to a slumber party.
Miller was driving her grandfather's (Nick Jensen) 1960 Studebaker Lark. It was described as beige or light tan with SD plates 19-3994.
The girls were at the hospital visiting Pearl Jensen, Miller's grandmother, until about 9:30 p.m. They were seen one more time that evening by three classmates in a church parking lot at Garryowen, just east of I-29 at the Akron exit.
The girls met three acquaintances � Pat Gale, Steve Glass and Mark Logterman, at the Garryowen corner. The three young men were waiting for friends to arrive. They were headed to a keg party located south of Union County Park near a large gravel pit.
They never arrived.
There has been speculation over the years that the girls, while driving in the wrong direction at night, may have wound up in the Missouri River.
The party was held at Gunderson's Chute, located several miles west of Burbank. A gravel road off the Burbank Road leads directly to the bank of the river.
Complicating the investigation immediately after the girls' disappearance was an unwillingness by many young people to talk to law enforcement.
Former Clay County Sheriff Arnold Nelson said the young people were protective. Talking to authorities was not a popular thing to do, especially when the questioning involved illegal parties, drugs and missing people.