By Sarah N. Lynch

WASHINGTON Former U.S. President Donald Trump faces a new legal challenge - this time from the government he used to lead - with charges for illegally retaining classified documents and other crimes expected to be filed next week in federal court in Miami.

The indictment of a former U.S. president on federal charges is unprecedented in American history, a case made more extraordinary by the fact that Trump is also the front-runner of his party for the Republican presidential nomination next year.

Trump faces seven criminal counts related to his treatment of sensitive government materials he took with him when he left the White House in January 2021, according to a source familiar with the matter.

Investigators seized roughly 13,000 documents from Trump's Mar-a-Lago estate in Palm Beach, Florida, nearly a year ago. One hundred were marked as classified, even though one of Trump's lawyers had previously said all records with classified markings had been returned to the government.

Trump has previously said he declassified those documents while president, but his attorneys have declined to make that argument in court filings.

"I AM AN INNOCENT MAN!" Trump wrote on his Truth Social platform on Thursday, after he announced that he had been indicted.

Trump lawyer Jim Trusty told CNN those charges include conspiracy, false statements, obstruction of justice, and illegally retaining classified documents under the Espionage Act. He said he expects to see the charging document by Tuesday, when Trump is due in court.

Trump is the first current or former president to face criminal charges.

It is the second criminal case for Trump, currently the leading candidate for the Republican nomination for the 2024 presidential election. He is due to go on trial in New York next March in a state case stemming from a hush-money payment to a porn star.

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Trump's legal woes have not dented his popularity with Republican voters, according to Reuters/Ipsos polling. His main rivals have so far lined up behind him to criticize the case as politically motivated.

Trump served as president from 2017 to 2021, and he has shown an uncanny ability to weather controversies that might torpedo other politicians. He describes himself as the victim of a witch hunt and accuses the Justice Department of partisan bias.

Special Counsel Jack Smith, who is heading the investigation, is also leading a second criminal probe into efforts by Trump and his allies to overturn his 2020 election loss to President Joe Biden, a Democrat.

He has been given a degree of independence from Justice Department leadership to pursue the politically sensitive cases.

Trump also faces a separate criminal probe in Georgia related to efforts to overturn his loss to Biden in that state.

Smith convened grand juries in both Washington and Miami to hear evidence, but has opted bring the case in the politically competitive state of Florida, rather than the U.S. capital, where any jury would likely be heavily Democratic.

Legal experts say that could head off a drawn-out legal challenge from Trump's team over the proper venue.

Trump is not the only top government official to draw scrutiny for retaining classified documents.

Attorneys for Biden and for Trump's then-vice president, Mike Pence, have said the Justice Department is also looking into their handling of sensitive papers they retained after leaving office.

Biden's documents dated back to his tenure in the U.S. Senate and as vice president. The Justice Department last week closed its investigation into Pence without filing any charges.

(Reporting by Sarah N. Lynch; Writing by Andy Sullivan; Editing by Noeleen Walder and Lincoln Feast.)