Mr. Sharp is apparently not so sharp. He carried out the attack from his home network. He connected directly for enough time that his bare IP was logged. The rest of the time, he carried out the attack using a commercially purchased VPN solution that was trivial to trace back to him via the purchase record. He lied to the FBI. (I have yet to understand why people talk to law enforcement instead of staying silent so a…
When the FBI knock at the door you totally do the whole "no comment/talk to my lawyer" thing. But what happens next if you're actually part of an investigation is they hand you a grand jury subpoena (which they were going to do anyway, even if you just talked willingly, because they have already gone to the trouble of asking a judge to issue one and have it with them by the time they ring your doorbell)
That subpoena is likely to require you to hand over any digital records you have related to the investigation (you can't plead 5th on that) and turn up at a time and place to be interviewed (you have to turn up, even if it's on the other side of the country eg in the Southern District of NY in Manhattan and you live in SF Bay Area). BTW I don't think people widely realize the government has the power to compel you to hand over EVERY piece of material you have on a given subject they are investigating - eg search and share anything from every email you have ever received since you signed up for GMail in 2004, etc.
You can plead 5th during the interview but if you have material information (or are actually guilty) and knowing they have all of the documentation subpoenaed and whatever other evidence from other subjects/targets/witnesses, it will likely help you at that point to be cooperative via guidance from your attorney. Remaining silent at that point is just going to leave you at the mercy of whatever other witnesses/subjects/targets convey and their own conclusions from the subpoenas.
If you are on a visa or green card you almost certainly can't plead the 5th because they can leverage your right to remain in the US.
So, that's why people typically talk to the FBI. It's not at the doorstep when they first engage you, it's once you have been compelled to participate.
Related/useful: https://www.natlawreview.com/article/you-received-grand-jury...
Source: happened to me a number of years ago, although I wasn't guilty of anything. Lawyered up, cooperated, no further action. Wasn't pleasant.
IANAL, not legal advice