The DOJ warned Elon Musk about his $1 million voting sweepstakes, CNN and The New York Times reported.

The gambit is part of Musk's efforts to mobilize swing-state voters.

Election law experts previously told Business Insider the contest was legally dubious.

Elon Musk's efforts to mobilize swing-state voters via a $1 million sweepstakes has the US Department of Justice watching.

The agency recently sent a letter to Musk's America super PAC warning that the contest could violate federal law, CNN and The New York Times reported Wednesday, citing people briefed on the matter. The 24Sight newsletter was first to report the news.

Neither Musk nor the DOJ immediately responded to Business Insider's request for comment.

At a Pennsylvania rally on Saturday, the Tesla CEO announced that he would give away $1 million each day through his PAC to swing state voters who signed a petition that affirms support for freedom of speech and the right to bear arms.

Paying people to register to vote is against the law, and violations of the law are punishable by fines of up to $10,000 or imprisonment of up to five years, BI previously reported.

The DOJ has sent similar warning letters to other businesses that have publicized voting promotions, The Times reported Tuesday.

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Several election law experts told BI earlier this week that Musk's latest gambit could spell legal trouble for the billionaire, who has thrown his support behind former President Donald Trump.

Rick Hasen, an election law professor at the University of California, Los Angeles, previously told Business Insider that the lottery could set "a very bad precedent," so he hopes the DOJ "would consider doing something."

"If Musk can do this, I think we will see many more payments and schemes like this in the future, upping the ante and changing the nature of our elections into ones where people focus on personal financial incentives rather than their best determination of whether and how to vote," Hasen said.

Matthew Alvarez, a partner at Rutan and Tucker, a law firm specializing in election law matters, told Business Insider that, while the payments may not be entirely legal, it's unlikely Musk will face criminal penalties for his actions.

Alvarez added that Musk could be looking to pick a fight with the DOJ because the "optics" could benefit Musk and the Trump campaign.

"The DOJ going after someone for encouraging political participation may very well be a fight that someone like Elon Musk wants," Alvarez said. "You know, 'Look at them coming after us. We're just trying to help people vote. We're just trying to get people registered,' right? That's a narrative there that at least sounds good in general."

Musk has already poured nearly $75 million into his America super PAC.