The Justice Department warned Elon Musk’s America PAC in recent days that his $1 million sweepstakes to registered voters in swing states may violate federal law, people briefed on the matter told CNN.

Musk, who has thrown his support behind former President Donald Trump and is spending millions of dollars supporting his candidacy, has publicized the $1 million prize by his political action committee aiming to increase voter registrations in hotly contested states.

Musk’s initial promise to pay prizes to registered voters immediately raised concerns from election law experts and some state officials who questioned whether it ran afoul of the law.

Federal law bars paying people to register to vote. The language of the petition currently promises $1 million prizes to people chosen at random for signing a petition in support of First and Second Amendment freedoms. But to sign the petition, you must be registered to vote in specific states.