NEW YORK (AP) -- President Joe Biden slammed Elon Musk for hypocrisy on immigration after a published report that the Tesla CEO once worked illegally in the United States. The South Africa-born Musk denies the allegation.

"That wealthiest man in the world turned out to be an illegal worker here. No, I'm serious. He was supposed to be in school when he came on a student visa. He wasn't in school. He was violating the law. And he's talking about all these illegals coming our way?" Biden said while campaigning on Saturday in Pittsburgh at a union hall.

The Washington Post reported that Musk worked illegally in the country while on a student visa. The newspaper, citing company documents, former business associates and court documents, said Musk arrived in Palo Alto, California in 1995 for a graduate program at Stanford University "but never enrolled in courses, working instead on his startup. "

Musk wrote on X in reply to a video post of Biden's comments: "I was in fact allowed to work in the US." Musk added, "The Biden puppet is lying."

Investors in Musk's company, Zip2, were concerned about the possibility of their founder being deported, according to the report, and gave him a deadline for obtaining a work visa. The newspaper also cited a 2005 email from Musk to his Tesla co-founders acknowledging that he did not have authorization to be in the U.S. when he started Zip2.

According to the account, that email was submitted as evidence in a now-closed California defamation lawsuit and said that Musk had apllied to Stanford so he could stay in the country legally.

Musk is today the world's richest man. He has committed more than $70 million to help Republican presidential nominee Donald Trump and other GOP candidates win on Nov. 5, and is one of the party's biggest donors this campaign season. He has been headlining events in the White House race's final stretch, often echoing Trump's dark rhetoric against immigration.

Trump has pledged to give Musk a role in his administration if he wins next month.

There was no immediate response to messages left with X and Tesla seeking Musk's comment.