Former US President Donald Trump was accused of handling classified documents after leaving the White House.
Trump, 76, faces seven charges, including unauthorized storage of classified files, US media reported, but further details were not clear.
It is the second indictment against Mr. Trump and the first federal indictment against a former president.
He is campaigning for a stunning return to the White House in 2024.
In a post on Truth Social, Mr. Trump said he was innocent and was summoned to appear in federal court in Miami on Tuesday.
“I never thought it possible for something like this to happen to a former president of the United States,” he wrote.
He added: “This is truly a dark day for the United States of America. We are a country in serious and rapid decline, but together we will make America great again!”
The BBC’s American partner CBS News reported a source as saying the indictment is related to the documents case and includes multiple charges.
Other US media outlets, including CNN and ABC News, said the indictment included at least seven counts.
An indictment allows a suspect to be prosecuted for crimes of which he is accused.
The Justice Department declined to comment and the indictment has not been released.
Special prosecutor Jack Smith has been reviewing evidence in the documents case since he was appointed to oversee it by Attorney General Merrick Garland in November.
Last year, Trump’s Mar-a-Lago resort in Florida was searched and 11,000 documents were seized, including about 100 marked classified. Some of them were labeled top secret.
Last week, it was reported that prosecutors obtained an audio recording of Mr. Trump admitting to keeping a classified document after he left the White House.
It is against US law for federal officials, up to and including the President, to remove or retain classified documents in an unauthorized location.
Mr. Smith, a former war crimes lawyer who is known as a tenacious investigator, is also overseeing a separate investigation into attempts to overturn the results of the 2020 election, which Mr. Trump lost.
Mr. Trump became the first former president to be charged with a crime in April after pleading not guilty to 34 counts of falsifying business records to pay hush money to a porn star.
He faces trial in the case in New York next year.