The existence of clearly marked classified documents in the trove — which has not previously been reported — is likely to intensify the legal pressure that Trump or his staffers could face, and raises new questions about why the materials were taken out of the White House.

While it was unclear how many classified documents were among those received by the Archives, some bore markings that the information was extremely sensitive — sometimes colloquially referred to as being “above top secret” — and would be limited to a small group of officials with authority to view such highly classified information, the two people familiar with the matter said.

The markings were discovered by the National Archives and Records Administration, which last month arranged for the collection of 15 boxes of documents from Trump’s Mar-a-Lago residence. Archives officials asked the Justice Department to look into the matter, though as of Thursday afternoon, FBI agents had yet to review the materials, according to two people familiar with the request.