Cowell was found guilty of murder, lying in wait, and attempted murder of Wilson's sister.
Cowell pleaded not guilty by reason of insanity.
The Alameda County DA released the following statement on Friday:
"Today brings to close one of the most tragic murder cases in recent memory. The horrific killing of Nia Wilson and the assault on her sister will haunt her family, loved ones and our community forever. Mr. Cowell will now serve the remainder of his life in prison. It is my hope that Nia's family feels that justice was served and can continue on the path of healing."
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