Democratic gubernatorial nominee Andrew Gillum on Thursday demanded that Florida TV stations stop broadcasting a “maliciously false” attack ad put out by the state Republican Party, warning that further broadcast of the ad would be intentionally defamatory.

In a cease-and-desist letter, lawyers for the Gillum campaign claims the ad was approved by his GOP opponent Rep. Ron DeSantis and makes “demonstrably false” claims. The 30-second spot focuses on an ongoing FBI public corruption investigation into the city government in Tallahassee, where Gillum serves as mayor.

“Andrew Gillum is running for Governor and also from the FBI,” the ad, which is cut through with images of police tape, states. It ends with a line saying Gillum is “not just radical, but corrupt.”

Gillum’s attorney writes that the candidate is neither the subject of the probe nor received any anonymous payments from anyone, as suggested.

“The Advertisement constitutes libel and slander of the worst sort,” the letter reads. “This letter shall serve as notice that any further publication or rebroadcast of the Advertisement by you will be intentional and made with actual knowledge of the maliciously false and defamatory statements contained therein.”

The FBI probe focuses on whether developers were able to influence Tallahassee development projects. Investigators have requested information from Adam Corey, a lobbyist and Gillum friend, but Gillum has not been named in any subpoenas.

The Tallahassee mayor has posted related records online, and pledged to work with the FBI.

The probe has been a major talking point for Florida Republicans in the close gubernatorial race.

Read the full letter below.