A 19 year old Brony walked into an Indianapolis FedEx place in 2021 and smoked 8 people and then himself. The guy had a lifetime of red flags including involuntary hospitalization and an interview with the FBI because his mom and others were worried once he both a rifle and mentioned suicide. Indiana has red flag laws but lack of follow up allowed him to get a new gun months later.
In March 2020, Hole's mother contacted the local authorities and warned them about her son's intent to die by suicide by cop and his purchase of a shotgun the day before, prompting an investigation to be opened. Police responded to the home and took him to a hospital. While being placed in handcuffs, an anxious Hole instructed the officers to turn off his computer, since he did not want anyone to see what was on it. An officer went upstairs to seize the shotgun and observed what he identified as white supremacist websites on Hole's computer in the process. Hole was placed in an "immediate detention mental health temporary hold" by the Indianapolis Metropolitan Police Department, but he was released after less than two hours. In April 2020, the FBI questioned Hole about the websites. The investigation was later closed due to insufficient evidence of any criminal violation or a racially motivated extremist ideology held by Hole, though the shotgun was not returned to him.
According to the police, Hole used two AR-15 style rifles in the shooting, and both of them were legally purchased from a licensed gun store in July and September 2020. Under Indiana's red flag law, Hole could have been prevented from making firearm purchases for at least six months after his temporary mental health detainment if a hearing had been scheduled with a judge fourteen days after the seizure of his shotgun. However, Marion County prosecutors decided to not schedule such a hearing, believing authorities had already achieved the law's objective since Hole's family did not want the seized shotgun back.
A lot of states passed red flag laws but local governments, for whatever reason, aren't making the most of it. Time and again we hear about mass shooters who were flagged or should have been flagged still able to legally buy guns. A big issue regarding the red flag laws is hostility to the idea of gun control among large segments of the population. There's also a nexus where right wing politics and mass shootings meet and I think police who are oftentimes right wing tend to look past red flag behavior in potential mass shooters.