I think it's pretty clear by now that there were major errors in the police response and they're trying to cover their blue asses.
That said, while it's tempting to blame them for those errors, the real blame goes to ridiculously easy access to weapons of mass slaughter and of course the shooter himself.
Also, it's probable that the shooter did most of the killings in the first few minutes which would make the long long delay mostly a moot point.