Former President Donald Trump was warned last year that holding onto classified documents after he was subpoenaed by the Department of Justice (DOJ) for their return to the National Archives could amount to obstruction of justice, according to records shared exclusively with The Guardian over the weekend.

"The previously unreported warning conveyed to Trump by his lawyer Evan Corcoran could be significant in the criminal investigation surrounding Trump's handling of classified materials given it shows he knew about his subpoena obligations," the outlet reports.

Noting special counsel Jack Smith's probe into why "Trump World" failed to return the requested materials, "prosecutors have fixated on Trump's valet Walt Nauta, after he told the Justice Department that Trump told him to move boxes out of the storage room before and after the subpoena. The activity was captured on subpoenaed surveillance footage, though there were gaps in the tapes," The Guardian reveals.

"The warning was one of several key moments that Corcoran preserved in roughly 50 pages of contemporaneous notes described to The Guardian on the condition of anonymity, which prosecutors have viewed in recent months as central to the criminal investigation," Washington correspondent Hugh Lowell writes.

"The notes revealed how Trump and Nauta had unusually detailed knowledge of the botched subpoena response, including where Corcoran intended to search and not search for classified documents at Mar-a-Lago, as well as when Corcoran was actually doing his search," Lowell continues, adding that "the notes ended up before the grand jury in Washington hearing evidence in the case after a US appeals court allowed attorney-client privilege to be pierced because judges believed Trump might have used Corcoran's legal advice in furtherance of a crime."

Lowell also explains from the notes that the room in which boxes filled with top-secret records were kept "might have been left unattended" during Trump World's search — which because it was incomplete prompted the Federal Bureau of Investigation to execute its own search warrant last August.

Furthermore, Trump was reportedly "irritated" with the subpoena as well as the "unusually detailed nature" of Corcoran's written accounts.

Reprinted with permission from AlterNet