Trump in midst of gathering storm of investigations. Mar-a-Lago document inquiry is one of many.

When former President Donald Trump disclosed that his Mar-a-Lago estate had been searched by the FBI Monday, the action prompted an immediate question: Of the multiple criminal investigations now pending, which one was the FBI pursuing?

Sources familiar with the matter have since confirmed that the unprecedented law enforcement action was related to a continuing federal investigation into allegations that Trump removed classified documents from the White House when he left office.

Yet the document probe is just one of a handful of federal and state inquiries that continue to press on the former president and some of his administration's closest allies.

Less than 48 hours after the Mar-a-Lago search, Trump's legal troubles appeared to only deepen Wednesday when he asserted his Fifth Amendment privilege against self-incrimination during questioning in a civil fraud inquiry led by the New York attorney general.

The case of the documents taken to Mar-a-Lago

The Justice Department earlier this year began its examination of a tranche of White House records, including some marked classified, that were sent to Trump's Mar-a-Lago resort.

Justice took the action after the National Archives in January obtained 15 boxes of presidential records that were being stored at Trump's club.

More: DOJ plans to investigate handling of White House records sent to Trump's Mar-a-Lago resort

According to the Presidential Records Act, the documents should have been transferred to the National Archives from the White House at the end of Trump's tenure.

The FBI's Monday search marked a dramatic escalation – and the most aggressive investigative action known to be taken against Trump in connection with any of the existing inquiries.

Under the law, any search would need to be authorized by a federal judge after finding probable cause that a crime had been committed and that evidence of the crime exists in the location to be searched.

The Justice Department's investigation into the 2020 election

In a separate inquiry, federal prosecutors have been questioning witnesses about the conduct of the former president as part of a larger inquiry into an effort to overturn the 2020 election.

Investigators have been examining attempts by the former president's allies to intervene in the election by substituting fake electors to tilt the vote in key states and to pressure former Vice President Mike Pence to block the certification of President Joe Biden's election.

More: Federal prosecutors questioning witnesses about Trump's conduct related to Jan. 6 inquiry

In June, federal investigators searched the home of former assistant attorney general Jeffrey Clark, who drafted a letter to officials in six states to overturn their election results. And authorities seized the cellphone of John Eastman, one of Trump's personal lawyers who developed a scheme to have former Vice President Mike Pence singlehandedly reject electors from states Biden won. Pence refused to carry out such a plan.

Top aides to Pence also have testified before a federal grand jury weighing the matter.

Atlanta-area prosecutors' probe of Trump's Georgia phone call

The Atlanta-area district attorney is leading a wide-ranging criminal investigation into alleged election interference by Trump.

Prompted by Trump's Jan. 2, 2021, telephone call to Georgia Secretary of State Brad Raffensperger in which the former president urged the state official to tilt the 2020 statewide vote in his favor, the investigation also includes an examination of a scheme to seat an alternative slate of Trump electors in Georgia to reverse Biden's victory there.

More: Georgia election investigation heats up. Giuliani, Eastman facing dates with grand jury

A number of Trump allies have been called to testify before a special grand jury, including the former president's personal attorney Rudy Giuliani, Eastman and Sen. Lindsey Graham, R-S.C.

Giuliani had been slated to appear Tuesday before the panel, but his lawyers asked that Giuliani's testimony be delayed because doctors have recommended that the former New York mayor refrain from air travel because of a heart condition.

Trump takes 5th in New York inquiry into the Trump Organization

New York Attorney General Letitia James is leading a civil inquiry focusing on whether the Trump Organization, the former president's namesake real estate business, claimed false property valuations in its dealings with lenders and taxing authorities.

A parallel criminal investigation was being led by the Manhattan district attorney's office, as part of an investigation that led to fraud charges against former Trump Organization chief financial officer Allen Weisselberg.

On Wednesday, Trump asserted his Fifth Amendment right against self incrimination when called for a long-sought deposition in the case, which he has cast as politically motivated.

"Under the advice of my counsel and for all of the above reasons, I declined to answer the questions under the rights and privileges afforded to every citizen under the US Constitution," Trump said in a statement.

Meanwhile, a  Trump spokesman lashed the attorney general for pursuing the case, calling James "a renegade and out-of-control prosecutor."

Last weekend, Trump protested the series of investigations during his address to the Conservative Political Action Conference.

He said Democrats are trying to prevent him from running again.

"Never forget everything this corrupt establishment is doing to me is all about preserving their power and control over the American people," Trump said. "They want to damage me in any form so that I can no longer represent the hardworking citizens of our country."

House Jan. 6 committee hearings elicits damning testimony

The most public – and perhaps damaging – of all the inquiries buffeting Trump and his former administration have played out in a series of public hearings hosted by a special House committee investigating the Capitol attack.

In those sessions, a parade of witnesses have described Trump's various roles in allegedly inciting the Jan. 6, 2021 riot, repeatedly calling on his Justice Department to open investigations into false claims of election fraud, while pressuring state officials to overturn 2020 election results based on those false fraud claims.

The committee has been sharing information it has uncovered with the Justice Department.

"This committee has shown you the testimony of dozens of Republican witnesses, those who served President Trump loyally for years," Rep. Liz Cheney, R-Wyo., said last month following the panel's eighth public hearing. "The case against Donald Trump in these hearings is not made by witnesses who were his political enemies. It is, instead, a series of confessions by Donald Trump’s own appointees, his own friends, his own campaign officials, people who worked for him for years, and his own family."

This article originally appeared on USA TODAY: Mar-a-Lago search: A breakdown of investigations involving Trump