Nia Wilson's Killer Has Been Arrested — Are They Going To Use Mental Defense?

Nia Wilson was standing on a Bay Area Rapid Transit station platform in Oakland, California, Sunday night when she was stabbed to death in an apparently unprovoked attack.

By Monday night, John Cowell, 27, had been arrested in connection to the stabbing, but days later, officials still haven't said what prompted the attack, which a police chief compared to a "prison yard assault."

The random murder of Wilson -- a black teenager described by her family as loving and selfless -- prompted nearly 1,000 people to march in Oakland in a vigil for Wilson early this week, according to an estimate by the Oakland police.

Some have claimed the attack was racially motivated, because Cowell is white, though the BART police and the Alameda County District Attorney's Office have said they don't have evidence to back up that claim. Cowell's family released a statement that said he had been struggling with mental illness.