The NYT is reporting that investigators served a subpoena to get the documents back in June but did not get full cooperation. I'm glad that they tried a lower key approach first before going nuclear.

And apparently, it isn't just a case of mishandling paper.

Two people briefed on the classified documents that investigators believe remained at Mar-a-Lago indicated that they were so sensitive in nature, and related to national security, that the Justice Department had to act.
So it was what was on the paper that was the problem, not the paper itself. This is moving away from "get Trump" and more towards something in response to his reckless disregard for anything approaching normal behavior.