STOCKTON (CBS13) - A Stockton man was shot and killed in the middle of the day, and neighbors say he may have been mentally ill. Police say they haven't ruled out self-defense.

Neighbors say the victim was shot in the stomach and died in the doorway of a home that had been sitting vacant for months. A few days ago, several men moved into the house as part of a sober living program -- something neighbors didn't warn them about.

"I mean a shooting, and they've been there a week!" said neighbor Caroline Kinnikan.

Neighbors on Flora Street are upset and outraged at the shooting, which happened just before 11 a.m.

A neighbor recounted what happened just moments before the shooting.

"I had just put the phone down from the police saying, 'you better get out here, this guy is going nuts on the corner and yelling at everybody,'" said Kinnikan.

They say one of the men who just moved in was acting erratic, and at some point, got into an argument with a man who lives across the street.

"And it wasn't more than three minutes later I heard two shots," said Kinnikan.

The landlord told CBS13 the owners of the home live out of town and had recently been leased the house to people in a sober living program.

"They don't need supervision, they're adults," said Aaron Cook, landlord.

"If they turned this into a halfway house and did not let anybody in this neighborhood know, that is going to be addressed," said Stockton Police Department's Ofc. Joe Silva.

Police said they questioned a 53-year-old man in regard to the shooting, but won't say if he was the neighbor.

"We are still trying to determine a motive, but one of the motives we are looking at is possible self-defense," said Silva.

No arrests have been made. The case has been turned over to the DA's office.