so what data sources would you use, and what data would you ignore, so that an accurate picture is established?
I would define the term homegrown terrorism and use all available reporting sources accordingly. Exhaustively vet all used sources. Weed out the reports which do not pass original definition. Vet the remaining stories again. Publish, citing all research including the used/unused reports and why.
Weeding out is hard to define because even though Mother Jones is left-leaning doesn't mean they do not have a valid point. I would weed out on a basis where the statement of events is easily corroborated with most, similar events. Like, a security officer negligently discharging his firearm is very different from a mass shooter.
Note the sources: