The Jefferson Parish Sheriff's Dept. has dispatched at least 2 cars to patrol the home of the man who shot and killed Joe McKnight ... TMZ Sports has learned.

We spoke with neighbors who live near Ronald Gasser in Gretna, Louisiana who tell us there is one car parked across the street from his home and another car patrolling the area. We're told the cars have been around for the past couple of hours.

We reached out to the JPSD for comment multiple times -- but they won't comment. 

TMZ Sports broke the story ... Gasser was released from police custody in the middle of the night and not charged with a crime. Cops have said he "definitely" fired the shots that killed McKnight. 

The investigation is ongoing.