The Justice Department has warned Elon Musk’s pro-Trump super PAC that its sweepstakes awarding $1 million to voters registered in swing states might be illegal.

The department issued a letter in recent days to Musk’s PAC America raising potential federal election law violations by the giveaway, sources familiar with the matter told CNN.

Musk has ramped up his support for former President Donald Trump ahead of the Nov. 5 general election, pouring $75 million into PAC America and appearing on stage at a rally for the GOP nominee.

In an effort to rouse support for Trump, particularly in swing states, he said on Saturday that he would award $1 million everyday until the election to a random individual who signed his petition and was also registered in one of seven key battleground states.

Immediately, many raised concerns about the ploy’s legality, as it’s against the law to pay people to register to vote.

Josh Shapiro—governor of Pennsylvania, one of the states whose voters can get paid—called for authorities to investigate the sweepstakes in a Sunday appearance on Meet the Press.

“I think there are real questions with how he is spending money in this race, how the money is flowing not just into Pennsylvania but apparently now into the pockets of Pennsylvanians,” he said. “That is deeply concerning.”

One legal expert, UCLA law professor Richard Hasen, called the billionaire’s tactic “clearly illegal.”

“Though maybe some of the other things Musk was doing were of murky legality, this one is clearly illegal,” wrote Hasen on his dedicated Election Law Blog.

Hasen said the practice is known as “vote buying” and involves offering to pay or accept payment for the registration to vote or voting.

However, Brad Smith, former chair of the Federal Election Commission, told the New York Times that Musk actually “comes out OK here” because he is paying people to sign a petition—which isn’t illegal—even though they have to be registered voters to do so.

America PAC has since the initial announcement reframed the sweepstakes as payment for a job rather than as a prize for registered voters who sign the petition. In a post on X featuring one of the winners praising Trump, the PAC wrote, “Every day until Election Day, a registered swing state voter who signs the petition will be selected to earn $1M as a spokesperson for America PAC.”

Musk has also said on X that winners “can be from any or no political party and you don’t even have to vote,” although he did not directly address the concern about whether the sweepstakes is paying people to register to vote.

At least two of the first three winners, though, have been registered Republicans who have already cast mail-in votes for Trump, Politico reported based on election records.