police officials reviewed the store's security video, they determined the incident was an "accident", not a result of being "irresponsible".
I think in police terminology the word "accident" probably has a legal meaning of "unintentional," whereas the word "irresponsible" has judgmental and moral ramifications. Police reports might be written using the former, but certainly not the latter.
Both are true - the killing was an accident, i.e., unintended, and the mother was irresponsible. She was careless - leaving a deadly weapon between herself and a child - apparently closer to the child. None of the known facts indicate otherwise and I would be seriously surprised if further investigation of this tragic but fairly straightforward incident reveals anything else. We shall see.
In the meantime, please don't chastise those of us who are genuinely heartsick and horrified by this appalling incident. Don't infringe my first amendment rights in service of your second amendment "rights."
In the meantime, NEVER underestimate a child's ability to mimic behavior he has seen previously demonstrated. Wise parents know this.