Earlier quoted context omitted.
Surely the bombs in London and Manchester were international news at the time, at least throughout the Western world? Next you'll tell me the average American is unaware of the 7 July bombs on the London Underground, the ETA bombs in Spain, Breivik in Norway or the sarin gas attacks in Tokyo.
The average American will have probably seen some coverage of those things, but for stuff that happened years ago that aren't closely tied to ongoing, present-day in-the-US issues, they won't recall many details.
That's part of why I read the BBC for world news.