The FBI said Diagne, 53, killed three people and injured 15 near downtown Austin on 1 March before dying in a gunfight with responding police.
Investigators said personal grievances tied to the Iran War, plus affinity for Iran and Ayatollah Ali Khamenei, were factors, but found no evidence linking him to a foreign terrorist group.
Diagne, a Senegal-born naturalized US citizen living near Austin, had not previously been investigated by the FBI and had legally bought the handgun and rifle used in the attack in 2017.
How did a distant war and a leader's death create a mass shooter in Austin overnight?
If lone attackers are inspired by foreign conflicts, can domestic security ever be truly guaranteed?
Beyond the tragedy, is the Iran War's economic fallout the bigger, unseen threat to global stability?