While mental health troubles could turn out to play a role in the case, it's unusual for symptoms to drive violence, says Edward Mulvey, a psychologist at the University of Pittsburgh School of Medicine who studies violence and mental illness.
The mother of the Iraq war veteran said he had been deeply shaken by seeing a bomb explode next to two friends while serving in Iraq in 2010, and relatives said he seemed different when he returned from service.
In November, he walked into an FBI field office in Alaska and said the federal government was controlling his mind and forcing him to watch Islamic State videos, authorities said.
While certain factors often show up in the history of mass murderers, like a history of failure, a tendency to blame others and social isolation, they also appear in the histories of people who don't harm anybody, Fox said.
The American Psychological Association says that while there's a small association between mental illness and violence directed at others, the overwhelming majority of people with serious mental illness don't pose a risk to others and should not be stereotyped as dangerous.