William Earl Talbott ll will make history when he appears in court in the state of Washington this week, charged with carrying out a double murder in 1987. It will be the first trial in which the prosecution will hinge on a revolutionary new crime-fighting technique, genetic genealogy. It is a case with legal and ethical implications that will be watched closely across the US. Experts believe the technique, which uses DNA submitted voluntarily to ancestry websites, could help police solve many of the 100,000 so-called cold cases – crimes that have been unsolved for years –across the US. Conventional police methods had brought the police no closer to identifying who was responsible for the killings of Jay Cook and Tanya Van Cuylenborg. The couple, childhood sweethearts from British Columbia in Canada, left their home for a 100-mile road trip across the US border to Seattle on November 18, 1987. Driving a bronze Ford van, they had planned to buy furnace parts for Jay's business. They never returned. Tanya's body was found on an isolated road 90 minutes from Seattle. She had been sexually assaulted and then shot in the head. Jay, who had been beaten and strangled, was found 60 miles away. His body was wrapped in a blue blanket; the only other clue was a sample of the killer's DNA. It was a classic cold case in which all leads had been exhausted. Investigators in Snohomish County, Washington, then tried a new approach, submitting the DNA found at the crime scene to a Virginia-based company, Parabon Nanolabs. Using a technique known as phenotyping, Parabon produced a composite image of the suspect aged 25, 45 and 65. Then, at the suggestion of CeCe Moore, the company's chief genetic genealogist, Snohomish investigators were asked if they wanted to try a different approach. This entailed using the GEDMatch DNA database, an ancestry website with just over a million entries. It produced two matches. One was Mr Talbott's second cousin, the other a half first cousin. There was no genetic link between the two, but a newspaper obituary tied the two strands together by a marriage. The identified couple had four children, three daughters and a son, William Talbott II, a man who was previously unknown to the police. Having been identified by Ms Moore, Mr Talbott was followed by police in the hope they would somehow get hold of DNA evidence matching that found at the crime scene. Unaware he was being tracked, Mr Talbott dropped a paper cup from the window of his truck. This was to prove a costly piece of litter. It was the only evidence that police needed to get a definitive match. It was a technique also used to identify Joseph James DeAngelo as the prime suspect in the famous case of the Golden State Killer – the nickname the man behind at least 13 murders, more than 50 rapes and 100 burglaries across California between 1974 and 1986. His full trial is due to start in August. For the families of Jay and Tanya, the arrest brought a measure of closure after more than three decades "Yesterday, the killer had his last sleep in his own bed, his last coffee break, his last day of freedom," said Jay Cook's sister, Laura Baanstra. "For my family and I, it is our first day without the weight, the burning, the hurting, that comes from not knowing who killed my brother Jay." Parabon says its techniques – phenotyping and genetic genealogy – have helped police solve 55 cold cases over the past year. The oldest cold case to use the technique was the murder and sexual assault of Susan Galvin in 1967. By examining a DNA sample from the crime scene, Parabon concluded the culprit was somebody with 16 per cent native American ancestry. The information led to the arrest of Frank Edward Wypych. "Without these types of ancestry clues," Ms Moore said, "many of our investigations would take months instead of weeks." Last month, police in Fayetteville, North Carolina, said the technology had helped them arrest Johnnie B Green Jr, who is accused of nine rapes between 2009 and December 2010. Within days, police in Terre Haute, Indiana, credited the technology for the arrest of Jeffrey Lynn Hand for the murder of Pamela Milam in September 1972. Parabon is understood to be looking at around 300 additional cases. However, there have been some concerns that people supplying their DNA voluntarily to ancestry websites have not been told that the material could be shared with law enforcement. In February the president of another ancestry website, FamilyTreeDNA, which marketed itself as a strict guardian of data, apologised for not disclosing that it had passed on material to federal investigators. However, Snohomish County Detective Jim Scharf makes no apology for the methods he used to arrest Mr Talbott and potentially crack a murder case which had been unsolved for 32 years. "He was never on any list law enforcement had, there was never a tip providing his name. If it hadn't been for genetic genealogy, we wouldn't be standing here today."