That's really not a counterexample to my point. This was a conspiracy involving over a dozen people. It's a plot that unraveled because someone leaked the details.

This is why technology has changed the balance. You don't need dozens of people to gather enough explosive material to pull this off today. And with fewer people involved, traditional detective work is harder, and those involved are less likely to leak details.

There are plenty of accidental detonations that killed hundreds which could have easily been caused by just one person as noted in another post. Though, wartime sabotage is not exactly terrorism.

Again, if you want to kill lot's of people disease, and fire where the most common historical examples. It's actually much harder for a lone person to kill thousands today than it was 500 years ago due to effective methods of firefighting.