Sixty-year-old Nellie Hicks of Newark and 48-year-old Theresa Pica of Hayward were attacked in their homes in 1972 and 1979, respectively.
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Both died from blunt force trauma and were sexually assaulted.
Police say the suspect, identified through DNA evidence, was Fred Farnham, a convicted rapist who died in 2007 at age 73.
Families members of both victims thanked investigators for their persistence.
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Dave Hicks showed a photo of his mother, Nellie.
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"I cannot express my gratitude enough for the police department and their dedication. All of them worked extremely hard to bring closure to our family," Hicks said.
Theresa Pica's daughters expressed their thanks in a statement, saying, "We've accepted the fact that whoever did this would never come to justice. Nevertheless, we thank God for you and your diligence and for never giving up."