The Justice Department is demanding identifying details about the senior Trump administration official who denounced the president in a New York Times Op-Ed last year under the byline Anonymous, according to a letter from a senior law enforcement official on Monday ahead of a forthcoming book by the still-unnamed writer.

Assistant Attorney General Joseph H. Hunt asked the book’s publisher and the author’s book agents for proof that the official never signed a nondisclosure agreement and had no access to classified information. Absent that evidence, Mr. Hunt asked that they hand over information about where the person worked in the government, and when.

“If the author is, in fact, a current or former ‘senior official’ in the Trump administration, publication of the book may violate that official’s legal obligations under one or more nondisclosure agreements,” Mr. Hunt wrote to Carol Ross of the Hachette Book Group, which is publishing Anonymous’s book, and to Matt Latimer and Keith Urbahn, the agents for the former self-described senior official.

Though a Justice Department official insisted that the letter was simply part of the typical fact-finding that officials do when executive branch employees plan to write about their government service, Anonymous’s representatives cast it as an effort to unmask their client, and a warning.