Again in Canada semi automatics are not as available -- and out mass shootings not as common or deadly.
We agree that availability is an issue --and shooters will use the fastest most firepower. While a machine gun is potential lethal to the shooter, these are not well trained people and even if the shooter dies, that does not mean bullets spraying will not also kill others. A trained person may not want a machine gun in close quarters but some random killer might - and the body count may still be higher even if they are at greater risk. I disagree that machine guns are less deadly -- I think they are so deadly that they endanger the shooter so we disagree there. If the shooter is also suicidal they might not even care. Also when it comes to shootings time is the issue -- time for authorities to arrive. The speed of a machine gun may attract.
As for the warnign signs -- these are usually seen after the fact. The reason is that there are too many warning signs available and knowing which ones are the priority is the problem. Hindsight is can provide lessons but the reality is that the resources are almost never there to identify warning signs, to follow them up, or to know which ones are the greatest threats. After a shooting there is huge energy, including from the media to follow up and look back to see what warning signs were there.
The biggest warning sign: shitloads of semi automatic guns in the populaiton -- will -- by the numbers mean many deaths. But the US does not want to take account of that. Reducing the number of guns reduces the number of people they ahve to worry about using them and the number they have to consider warning signs from.
I have to add that I have not seen data on these warning signs but looking forward I suspect that they are out of proportion to the number of resources to process them.