
A more algorithmic rule is found in APA style: https://blog.apastyle.org/apastyle/2016/10/hyphenation-stati...
Edit: NPR also uses the Associated Press Stylebook and related resources.
I'm fine with that definition and, thereby calling this incident terrorism. However the word terrorism is used strictly in the Orwellian sense by the media. It is almost always used for violence against us, never committed by us, and usually for militant Muslim groups activities.
If we think about it, the biggest perpetrators of terrorism are armies of large, powerful countires. But it is never depicted that way.