system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When the media reports that the "FBI questioned x for ties to terrorism"- how does that work? Do they just knock on the door?
gpt
Typically it means they've either been arrested for something similar, or that they've, indeed, just had the police/FBI etc around to ask some questions. Quite literally, knocking on the door and asking to come in.

This usually means that there's some suspicious link, but not necessarily something that means you're a terrorist. Eg they've not been caught carrying a bomb and wearing an "I <3 Osama Bin Laden" t-shirt.

Typically, it means they're friends/acquaintances with someone else who's under investigation for a similar reason: often in conjunction with something else that may or may not look slightly suspicious.

eg if you stopped to chat in the mall with someone who's under surveillance, and then you buy a bunch of fertiliser, the FBI might stop round to enquire about your gardening habits.