Donald Trump dove into the Florida gubernatorial race on Monday morning with an explosive allegation: the Democratic candidate, Tallahassee mayor Andrew Gillum, is a "thief" who runs a corrupt city.

Gillum, who is running against former Republican Rep. Ron DeSantis, responded on Twitter, calling Trump "weak." A senior aide for Gillum called the claim "a disgusting lie."

The hotly contested race has been dominated by allegations of wrongdoing on both sides, but DeSantis has hit Gillum hard over an ongoing FBI investigation into Tallahassee's city government.

Let's look at what is known about the case.

What is the FBI investigating?

Asked about Trump's comment at the White House press briefing, Press Secretary Sarah Sanders said the president made the claim because Gillum is "under FBI investigation," and declined to comment further.

There's no evidence that Gillum is personally being investigated, and he has not been charged with any wrongdoing. There is a federal probe into Tallahassee's city government.

The FBI does not discuss pending investigations and charges haven't yet been filed, but extensive reporting by the Tallahassee Democrat has revealed some details. Subpoenas and other documents suggest a city commissioner and a lobbyist friend of Gillum’s, Adam Corey, are at the heart of the investigation into business and potential corruption between officials and developers. Corey also served in a volunteer capacity as a treasurer for Gillum's mayoral campaign, according to the Tampa Bay Times.

Gillum has said he met with FBI agents and that agents told him he is not the focus of the investigation, though the FBI does not typically reveal such information publicly.

Still, Gillum has struggled to explain away his involvement with an FBI agent posing as a businessman seeking to do business with the city, and his travels with lobbyist friends.

What did Trump mean when he called Gillum a "thief?"

When asked about his meaning in an interview with Fox News Channel's "The Ingraham Angle" that aired Monday night, Trump said, “The FBI offered him tickets at $1,800 apiece and he took them — he took a trip with the same FBI agent— I guess he was posing as a developer or something — the man stone cold took this stuff."

In 2016, Gillum hung out in New York City with undercover FBI agents posing as businessmen, who arranged tickets to the Broadway musical "Hamilton," among other activities, for the mayor, his brother, and Corey.

Gillum said his brother gave him the "Hamilton" ticket, but text messages released amid the investigation revealed that Gillum was aware of the origin of the sought-after ticket and that Corey arranged for a hotel room for Gillum.

What's more, the text messages also appear to show that the same undercover FBI agent paid for expenses at a political fundraiser at Corey's house, though the donation was not listed on campaign finance reports and the campaign has denied that they received any donations from the undercover FBI agent.

Are there any other ethics investigations?

The FBI investigation has prompted a separate state ethics probe concerning other travels, including a trip to Costa Rica with a lobbyist friend who later received $2.1 million in public funds for a Tallahassee restaurant.

A city-funded plane trip that appears to have involved campaign-related activity has also raised questions, too.

Gillum has maintained he paid his own way and denied allegations of wrongdoing.