Fifteen years after JonBenet Ramsey was killed in Boulder, police say the case is still considered an open investigation. Saturday would have been JonBenet's 21st birthday.Boulder police told 7NEWS this week that more than 150 DNA samples have been tested in the investigation. More than 140 people have been investigated as potential suspects, but none could be linked to the crime.Boulder police took the investigation back from the district attorney's office in 2009 saying they would apply new technology and expertise in hopes of solving the case.At the time, newly elected District Attorney Stan Garnett agreed that police should lead the investigation again."District attorneys prosecute and police investigate," Garnett said. "I have carefully reviewed the state of the evidence in the Ramsey case, meeting with many people both inside and outside of my office. Based on the state of this evidence, no charges can be filed against anyone in this case at this time. Possible leads come in on a regular basis and the Boulder police should investigate these leads. I, in consultation with my staff, will make any filing decision based on the evidence."In 2009 Boulder police formed a task force to provide ideas and direction for the investigation, but a spokeswoman told 7NEWS the task force is not an ongoing group.JonBenet, a Boulder beauty pageant contestant, was found dead in the cellar of her parent's home on Dec. 26, 1996. She had been bludgeoned and strangled. Her parents went on CNN several days later to talk about their daughter and the case eventually attracted worldwide attention.A former prosecutor said Ramsey and his wife, Patsy, were "under an umbrella" of suspicion, but authorities cleared them in 2007. Authorities said new DNA tests pointed to an unknown outsider.JonBenet's mother, Patsy Ramsey, died from cancer in 2006. JonBenet's father, John Ramsey, remarried in July.