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."
RELATED STORIES:
- Heartbreaking testimony from Nia Wilson's sister in deadly BART stabbing trial
- Man accused of killing BART rider Nia Wilson thrown out of court during trial opening statements
- Judge rules Oakland BART stabbing suspect competent to stand trial
- 'He was wiping off his knife': BART stabbing victim recalls horrific attack that killed sister
- The judge threw the defendant John Lee Cowell out of court after he had more than one outburst.