The FBI recovered more than 20 boxes of items - some labelled 'top secret' - during a search of Donald Trump's Florida home, US court records show.
Details from a warrant that has now been unsealed show some items related to defence information.
The warrant said the probe involves alleged violations of the Espionage Act.
A binder of photos and a handwritten note are among the items seized, along with a potential presidential record.
Among the 20 boxes removed were 11 sets of classified documents.
According to Fox News, one set was marked as "various classified/TS/SCI documents" - referring to top secret and sensitive information.
The FBI searched Mr Trump's Mar-a-Lago estate earlier this week.
The warrant described the property as "a mansion with approximately 58 bedrooms, 33 bathrooms, on a 17-acre estate".
It said officers could search the "45 Office" - a reference to Mr Trump's status as the 45th American president - "all storage rooms, and all other rooms or areas within the premises used or available to be used by FPOTUS" (Former President of the United States).
The unsealed warrant allowed for "all physical documents and records constituting evidence, contraband, fruits of crime, or other items illegally possessed" to be removed, including:
- Any physical documents with classification markings, along with any containers/boxes (including any other contents) in which such documents are located, as well as any other containers/boxes that are collectively stored or found together with the aforementioned documents and containers/boxes
- Information, including communications in any form, regarding the retrieval, storage, or transmission of national defence information or classified material
- Any government and/or Presidential Records created between 20 January 2017, and 20 January 2021
- Any evidence of the knowing alteration, destruction, or concealment of any government and/or Presidential Records, or of any documents with classification markings
The former US president claimed his home was "raided" and "under siege" and that agents broke open a safe.
Speaking to Newstalk earlier this week, Mr Trump's former chief of staff Mick Mulvaney said such a warrant would have had to reach certain thresholds.
"They have to be able to show a judge that there's probable cause that a crime has taken place, probable cause that there's evidence around that crime at a particular place, and that there is a likelihood that that evidence would disappear if the Government can't get its hands on it right away.
"That's apparently what the Department of Justice convinced a judge of, which is what gave them the right to go into the president's house".
He added: "Either it's the most egregious political assassination attempt by my own Government against a political leader in history - or it's the most significant criminal investigation into a president in history.
"Keep in mind: not even Nixon had his home raided".
Additional reporting: Jack Quann