In 1976, a young couple was killed while camping. In 2019, police tricked the suspect into handing over his DNA

The engaged couple was shot dead while camping in the summer of 1976.

In the summer of 1976, David Schuldes and his fiance, Ellen Matheys, were enjoying a weekend camping trip in Silver Cliff, Wisconsin, when they were shot dead, according to court documents.

The double murder went unsolved for decades.

This year, police zeroed in on the suspected killer, Raymand Vannieuwenhoven, and tricked him into handing over his DNA, according to documents.

The 82-year-old suspect, who maintains his innocence, was ordered on Wednesday to face trial.

A mysterious crime

On July 9, 1976, Schuldes and Matheys left their separate homes and headed to McClintock Park to camp, according to the criminal complaint against Vannieuwenhoven.

Schuldes, 25, worked at the Green Bay Press-Gazette, while Matheys, 24, worked at the University of Wisconsin-Green Bay library, the Green Bay Press-Gazette reported.

After setting up their tent, Schuldes was shot in the neck, the criminal complaint said.

The suspect then sexually assaulted Matheys in a wooded area; as she was getting dressed, he shot her twice in the chest, according to court documents.

When Matheys' body was found, semen was recovered as evidence, documents said.

But no arrest was made.

Following the DNA

In the 1990s, as DNA technology emerged, the unknown suspect DNA from the sex assault was uploaded to the Combined DNA Index System -- a law enforcement database known as CODIS -- but there was no match, documents said.

The years continued to tick by, and in 2018, investigators began to work with Parabon Nanolabs, which specializes in genetic genealogy.

With genetic genealogy, an unknown killer's DNA from a crime scene can be identified through his or her family members, who voluntarily submit their DNA to a genealogy database. This allows police to create a much larger family tree than using law enforcement databases like CODIS, in which an exact match is needed in most states, according to genealogy expert CeCe Moore.

In April 2018, the suspected "Golden State Killer" became the first public arrest through genetic genealogy. Since then over 50 suspects have been identified through the technology, according to Moore.

Moore is the chief genealogist with Parabon, the lab which worked on the Silver Cliff case.

A genealogist narrowed "down a suspect pool to a specific family with ties to the Green Bay, Wisconsin, area," according to the amended criminal complaint filed on Tuesday. "The genealogist thought the suspect could be one of their four sons or four of their grandsons."

Raymand Vannieuwenhoven was the third son to be surveilled. He is from Lakewood, Wisconsin, about 20 miles from Silver Cliff.

The first son was surveilled in January 2019. Investigators took a bag out of his trash and sent socks, a bandage and inhaler for DNA testing, court documents said. But his DNA was not a match to the crime scene DNA, court documents said.

The second son was surveilled in February 2019. According to documents, he would occasionally meet up with a neighbor, a retired sheriff's detective, for coffee. The detective kept the man's coffee cup and handed it over to investigators, documents said. His DNA did not match either, documents said.

Finally, Vannieuwenhoven was surveilled in March 2019. Two officers went to his home and asked "if he would do a brief survey about policing in the various townships," according to documents.

After he completed the survey, the officers asked Vannieuwenhoven to seal it in an envelope, the documents said.

Vannieuwenhoven's saliva from the envelope was sent to the state crime lab where it was found to be a match to the semen from Mathey's sex assault, the documents said.

"Genetic genealogy has served as a highly reliable pointer to investigators in dozens of cases," Moore, the chief genealogy with Parabon Nanolabs, told ABC News. "But since it is only used as a tip, surreptitious DNA collection is utilized to confirm or refute any theory provided by genetic genealogy research before an arrest can be made. For this reason, in many cases, detectives have been devising highly-creative ways of collecting DNA from persons-of-interest. "

'Our client maintains his innocence'

Vannieuwenhoven was charged in March with two counts of first-degree murder and one count of first-degree sexual assault, according to court records.

The sexual assault charge has since been dismissed based on a previously argued defense motion, defense attorney Lee Schuchart told ABC News.

Vannieuwenhoven appeared in court on Wednesday and was bound over for trial on two counts of first-degree murder.

He is set to be arraigned on July 1 "at which point he will enter not guilty pleas," Schuchart told ABC News via email on Wednesday.

"Our client maintains his innocence," he said.