Using your timeline of mass shooting increases, an immediate reason to consider should be the assault weapon law expiring in 2004. Data would back that up. We haven't had the same laws forever. https://ohiocapitaljournal.com/2022/06/15/did-the-assault-weapons-ban-of-1994-bring-down-mass-shootings-heres-what-the-data-tells-us/
Thanks for the clarification. I see they updated the headline on the website to now say original.