What we do and don't know about the FBI search of Trump's home

When Donald Trump's Florida home was searched by the FBI, it unleashed a political firestorm unlike anything in recent memory.

The release of the search warrant shed light on why it happened. Now, a judge is weighing up whether to unseal a highly sensitive document that could give a clearer sense of how the search at Mar-a-Lago was justified.

But this unprecedented story is complicated and many questions remain. Here's what we know so far.

In short, because the US Department of Justice suspects the former president may have committed a crime.

The search warrant shows FBI agents gathered evidence on 8 August as part of an investigation into whether Mr Trump improperly handled government records by taking them from the White House to Mar-a-Lago.

It's worth noting here that US presidents must transfer all of their documents and emails to a government agency called the National Archives.

And earlier this year, that agency said it had retrieved 15 boxes of papers from Mar-a-Lago which Mr Trump should have handed over when he left the White House. It said they included classified information and asked the justice department to investigate.

To obtain the search warrant, prosecutors had to persuade a judge that they had probable cause to believe a crime may have taken place. We also know that the effort to seek a search warrant was signed off by the head of the justice department - the attorney general - who is the country's top legal official.

Twenty boxes worth of material, according to an inventory released alongside the search warrant.

The FBI took 11 sets of classified files in total, including four that were labelled "top secret". Three sets were classified as "secret documents" and three were "confidential".

The cache also included files marked "TS/SCI", a designation for the country's most important secrets that if revealed publicly could cause "exceptionally grave" damage to US national security.

Some of these files were only meant to be kept in secure government facilities, according to court documents.

But the court records do not indicate what information these documents could contain, and there is much we do not know about the items on the inventory.

For example, other materials taken include a binder of photos, a handwritten note and unspecified information about the "President of France".

The former president has been vocal about the FBI search and has repeatedly denied wrongdoing.

He said the documents taken by the agents were "all declassified" and had been placed in "secure storage". He said he would have turned them over if the justice department had asked.

However, the New York Times reported that the department had sought additional documents from Mr Trump in the spring that it believed may have been in his possession. It said it was not clear why the recovered files were not handed over at this time.

Mr Trump has also offered shifting explanations as to why the documents were at Mar-a-Lago in the first place.

He has suggested that evidence may have been planted, falsely claimed that President Obama retained classified files, and repeatedly insisted that all the documents were declassified.

It is questionable whether it matters - in a legal sense - whether the documents had been declassified. More on this later.

There are three main categories of classified material - confidential, secret and top secret. These are given depending on the extent to which officials deem the public release of the material would damage national security.

When materials are classified, they are marked as such and only certain people - those who have passed the relevant level of security vetting - should be able to view them. There are also rules dictating how classified information is transported and stored.

Some senior officials do have the power to declassify documents. The president, too, has the authority to do this, but would usually delegate the task to those who have direct responsibility for the material.

It's worth noting that documents related to nuclear weapons cannot be declassified by the president, as they fall under a different law, according to a security specialist who spoke to the Washington Post.

As the BBC's US partner CBS reports, the president cannot declassify documents by simply saying so. A written memo would usually be drafted and signed by the president, before a consultation process takes place with the relevant agencies.

Once a final decision is made, the old classification level would be crossed out and the document marked as "declassified on x date".

It is unclear whether Mr Trump followed the typical process with the documents recovered from Mar-a-Lago.

In a legal sense, probably not.

That's because prosecutors are investigating three potential crimes. These are:

Crucially, none of the three criminal laws in question actually depend on whether or not the files were declassified.

This means it is uncertain whether Mr Trump's argument about declassification would hold up in court or even be considered relevant by a judge.

There are also elements of this story beyond declassification that could prove relevant. There are laws designed to prevent national security documents from being mishandled, for example, while the Presidential Records Act dictates that presidents risk committing a civil offence if official records are not preserved.

All attention now is on a highly sensitive document - the affidavit - which outlines the evidence the justice department had gathered in order to obtain the search warrant.

That's because a group of news organisations have argued the release of that document is in the public interest. The justice department, however, opposes its release, saying this could jeopardise its investigation.

A federal judge has heard both arguments, and concluded he was "inclined" to unseal some of the affidavit.

He instructed the justice department to redact the document in a way that would not undermine its ongoing investigation and gave it a deadline of 25 August to do so. That means - if it is made public - some sensitive elements will likely be removed.

But it could still contain some dramatic new details about the search.

None of this means the affidavit will definitely be released. That decision still rests with the judge - so it remains to be seen what happens next.

Mr Trump, meanwhile, has not been charged with wrongdoing. It remains unclear whether charges will be brought as a result of the investigation.