South African-born billionaire Elon Musk worked illegally in the United States for a brief period in the 1990s while building a startup company, the Washington Post reports.

Musk arrived in Palo Alto, California in 1995 to attend Stanford University, but never enrolled. Instead, he developed the software company Zip2, which was sold in 1999 for about $300 million, according to the US newspaper.

Two immigration law experts cited by the Washington Post said Musk had to be enrolled in college in order to work as a student.

Musk did not respond to requests for comment sent to four of his companies — SpaceX, Tesla, Social Enterprise X and The Boring Company — nor did Musk's attorney, Alex Spiro.

In a 2020 podcast cited by the Post, Musk said, “I was legally, only supposed to do student work. I was allowed to work, in support of everything.”

The Washington Post quoted two of Musk's former colleagues who recalled that the latter obtained a US work permit around 1997.

Musk is supporting Republican presidential candidate Donald Trump in the November 5 election. Trump has portrayed immigrants as invaders and criminals for years, and during his 2017-2021 presidential term took tough steps to curb legal and illegal immigration.

He promises the largest deportations in US history if re-elected.