Trump, who plans to run again in the 2024 presidential election, said he was summoned by a court in Miami on Tuesday. "I never thought it possible that something like this could happen to a former President of the United States," he wrote.
Several US media outlets unanimously reported on Wednesday that prosecutors from special counsel Jack Smith's team had informed Trump's lawyers that the ex-president was a personal target of their investigation.
Trump is already the first ex-president in US history to be indicted: In March, Manhattan prosecutors charged him with paying hush money to porn actress Stormy Daniels ahead of the 2016 presidential election.
The US federal police FBI seized around 11,000 documents, including many with classification levels, during a raid on Trump's private home in Mar-a-Lago in the US state of Florida last August. Trump took the documents from the White House to Mar-a-Lago at the end of his term in January 2021.
Presidents are required by law to turn over official records to the National Archives after the end of their term of office. In addition, there are strict legal rules for dealing with secret documents, including a law on espionage.
In November, Attorney General Merrick Garland appointed a special counsel, Jack Smith, to take over the investigation into the affair. Smith is also investigating Trump's criminal liability in connection with the Jan. 6, 2021 Capitol storming.