Prosecutors dropped charges Friday against the sole remaining suspect in a triple murder in Yakima two years ago, dealing yet another blow to a case that has stymied investigators and prompted a judge to appoint a special prosecutor to investigate potential misconduct.

In a statement, Yakima County prosecutors said they had insufficient evidence to prosecute Tracy Culton in the February 2011 deaths of a man, his wife and 98-year-old mother. Bill and Pauline Goggin and Bill's mother, Bettye Goggin were found beaten to death at their home in a gated community west of Yakima.

Yakima County Superior Court Judge Ruth Reukauf dismissed the charges without prejudice, which means they could be refiled, the Yakima Herald-Republic reported.

Culton offered details and acknowledged her involvement in the murders to third parties, but investigators interviewed dozens of witnesses who supported her alibi. Culton also passed a polygraph test when asked directly about her involvement in the murders, Prosecutor Jim Hagarty said in a statement.

Within the past week, Washington State Crime Lab test results on forensic evidence from the crime scene also failed to implicate Culton, Hagarty said.

"Throughout the year and a half of investigation in this case, it has been made clear that Ms. Culton was not involved in the planning or participation in the triple homicide," the statement said, adding that authorities will continue to investigate this "tragic case" and are hopeful that justice will be served.

Family and friends of the victims were stunned last fall when prosecutors dropped murder charges against the primary suspect in the case, Kevin Harper, in a plea deal.

Harper pleaded guilty in October to one count of unlawful possession of a firearm and one count of possession of stolen property and was sentenced to just over seven years in prison.

Prosecutors said then that they did not believe murder charges against Harper could be proven. That decision followed the appointment of a special prosecutor to investigate whether authorities eavesdropped on jail calls between Harper and his attorneys.

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Information from: Yakima Herald-Republic, http://www.yakimaherald.com