USA: Elon Musk is being investigated by the FBI in connection with a $44 billion acquisition of Twitter, according to a court filing made public on Thursday.

While the filing said he was being investigated, it did not specify the nature of the investigation or which federal officials were looking into him.

Twitter, which sued Musk in July to force it to close the deal, said Musk's lawyers claimed "investigative privilege" by refusing to turn over the requested documents.

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Musk's lawyers provided a "privilege log" identifying the documents to be withheld in late September. Twitter said the log included a May 13 draft of an email to the US Securities and Exchange Commission (SEC) and a slide presentation to the Federal Trade Commission (FTC).

"This 'hide the ball' game must end," the company said in the court filing.

The SEC had previously questioned Musk's comments about the Twitter acquisition. The SEC asked Musk in April whether the disclosure of his 9% Twitter stake was late and why it indicated that he intended to become a passive shareholder. Musk later amended his disclosure to reflect that he was an active investor.

In a letter sent in June, the SEC asked Musk whether he should have amended his public filing to reflect his intention to suspend or drop the deal.

The SEC and FTC did not immediately respond to requests for comment.

Musk's lawyer, Alex Spiro, told Reuters that Twitter's court filing was a "misdirected move ... Twitter executives are being investigated by the FBI."

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Twitter did not respond to Spiro's statement. When asked by Reuters about the nature of Musk's investigation, he also declined to comment.

The filing comes as the months-long saga of Musk's proposed Twitter takeover continues, during which the billionaire proposed a takeover, walked out of the deal, and then sued Twitter to keep its word.

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Musk's team then said it would proceed with the transaction a few days before the trial to force the purchase. Musk is reportedly in the process of securing financing to complete the purchase.