The ex-Marine and New York City native who police say shot two co-workers at a New Jersey supermarket before killing himself had tweeted thoughts about killing other people three years ago.
While stationed in California, Terence Tyler asked on Twitter "Is it normal to want to kill all of your coworkers?"
A few months later he also tweeted, "I'm starting to see why people go on killing sprees."
Tyler was discharged in 2010.
Family members say he suffered from depression and had never gotten over his mother's death five years ago.
Police say the 23-year-old left his shift at a Pathmark in Old Bridge early Friday morning, then returned in military clothing with a handgun and an AK-47.
They say he opened fire, killing 18-year-old Christina LoBrutto and 24-year-old Bryan Breen before turning the gun on himself.
About a dozen other workers escaped unharmed.
Authorities say Tyler only started working at the store a few weeks ago.