Birkbeck was on probation until 2029 in the second-degree murder of his estranged wife. He was also convicted of manslaughter in Massachusetts in 1962, according to New Hampshire news outlets.
Neighbors told Bay New 9 that they had no idea of Birkbeck's background. "It's just shocking because you never know who's living right next door to you," one said.
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At the crime scene Wednesday, Pasco Sheriff Chris Nocco held a press conference, lamenting the "horrific day" for the victim's family. "We just hope that people pray for her."
As for Birkbeck, the sheriff said: "No other way to say it. He's a killer."