The Washington Post goes back to the prior narrative surrounding the Mar-a-Lago documents saying,

a document describing a foreign government’s military defenses, including its nuclear capabilities was found by FBI agents who searched former president Donald Trump’s Mar-a-Lago residence and private club last month.”

…. with Devlin Barrett adding the usual, “according to people familiar with the matter” sourcing.

Keeping in mind the traditional relationship that exists between specific media outlets and their deep state sources, the most obvious source for Devlin Barrett is the DOJ National Security Division (DOJ-NSD), FBI Counterintelligence division, or larger intelligence community (IC).  However, what is also obvious from the way the documents are described is a narrative framework to make the innocuous seem looming.