Robert Hawkins, who killed eight people then himself on a gun rampage through a Nebraska mall, had "lots of emotional problems", says the woman who took him in after he left home.
Hawkins was very quiet, say those who knew him
Debora Maruca Kovac, said he was withdrawn - "like a lost pound puppy that nobody wanted".
She said Hawkins, 19, was not on any medication for mental illness but was treated for depression in the past.
He had also recently broken up with his girlfriend, and then lost his job.
The Omaha World Herald newspaper reported that he had been arrested on Friday for being a minor in possession of alcohol.
The newspaper also said he was sent to jail for seven days in 2005 on charges of disorderly conduct.
Hawkins had been living with the Maruca Kovac family for about a year-and-a-half after befriending her sons.
It is not clear why he left home, although it is understood that his mother lived nearby.
While he has his troubles, the teenager was gentle and loved animals, Debora Maruca Kovac said.
"I felt sorry for him. I let him stay, and we tried to get him on his feet," she told CNN.
She said he earned a high-school equivalency degree after dropping out of Papillion La Vista High School, and got a driver's license after moving in with them.
He started working at a McDonald's restaurant near their home about five months ago.
She said he liked to listen to music and play video games - "normal teenage stuff" - but he had a drinking problem and would occasionally smoke marijuana in his bedroom.
"He was a very helpful young man, but he was quiet," she said.
"He didn't cause a lot of trouble. He tried to help out all the time. He was very thankful for everything. He wasn't a violent person at all."