Former US President Donald Trump's second indictment concerns accusations that he mishandled classified documents in a case believed to be his most serious legal threat as yet.

Trump announced the indictment on his Truth Social platform, reiterating that he was "innocent." He said he had been summoned to appear at the federal courthouse in Miami on Tuesday. It marks the first time a former president has been indicted in a federal court.

Though the indictment remains sealed, media reports suggest the former president has been indicted on seven counts. They relate to his handling of classified documents and obstruction of justice.

The case adds to Trump's legal woes and further complicates his 2024 presidential bid.

Earlier this year, he was indicted in March in New York in a state case over an investigation into hush money paid to adult film star Stormy Daniels. He is due to face trial next March, in the midst of the primaries.

How did the classified documents case begin?

The case has been brewing since shortly after Trump lost the 2020 election race and had to move out of the White House.

In the spring of 2021, National Archives and Records Administration officials realized important documents from Trump's tenure were missing from their collection.

The Presidential Records Act deems all White House documents the property of the US government, meaning they must be preserved.

Archives officials reached out to Trump representatives, one of whom told them in December of the same year that presidential records had been found at Trump's Mar-a-Lago estate in Palm Beach, Florida.

In January 2022, the National Archives retrieved 15 boxes of documents from Mar-a-Lago. They later told the Justice Department the boxes contained "a lot" of classified material.

However, that did not account for all the missing documents. In May 2022, Trump was served an FBI and Justice Department subpoena for the remaining classified documents in his possession.

Weeks later, investigators visited the Florida estate and were given some three dozen documents with a sworn statement from Trump's lawyers attesting that the requested information had been given back.

Why did the FBI raid Mar-o-Lago?

However, the assertion was later revealed to be false.

In August 2022, federal officials aided with a search warrant raided a storage room and an office in Mar-o-Lago, seizing over 33 boxes that contained thousands of documents. The boxes included 100 classified documents.

Since he left office in January 2021, some 300 classified documents have been recovered from Trump, including some at the top secret level.

The president did not deny holding onto the classified documents. He defended his actions, saying he had them declassified back when he was in office, yet without providing evidence to support this claim.

According to US law, incumbent presidents can declassify information, but that authority lapses once they leave office.

What are the charges against Trump?

It is not yet clear what charges the former president faces.

Yet, the seizure of the documents from Trump's Florida estate contravenes three different laws, which all deal with mishandling federal government records. The laws apply regardless of whether the records are still classified.

The first law cited prevents unauthorized possession of national defense information and is punishable by up to 10 years in prison.

The other two laws, which prohibit concealing or destroying official US documents, are punishable by up to three and 20 years in prison, respectively.

Trump's lawyer Jim Trusty told CNN after the indictment that charges his client faces include conspiracy, false statements, obstruction of justice, and illegally retaining classified documents under the Espionage Act.