The Washington Post reported Saturday that South African-born billionaire entrepreneur Elon Musk worked illegally in the United States for a brief period in the 1990s while building a startup company.

The US media reported that Musk arrived in Palo Alto, California in 1995 to study at Stanford University, but never enrolled in his graduate program there.

Instead, he grew the software company Zip2which he sold in 1999 for about $300 million.

Two immigration law experts cited by the Washington Post said Musk would have to enroll in a full-time course of study in order to maintain his work permit as a student.

Musk did not respond to requests for comment sent to four of his companies, SpaceX, Tesla, X and The Boring Company.

Musk’s lawyer, Alex Spiro, did not comment either.

The Washington Post cited two former Musk colleagues who remember the billionaire getting a US work permit around 1997.

Musk is endorsing Donald Trump in the November 5 election. Trump has for years portrayed immigrants as invaders and criminals. During his 2017-2021 presidency he took strict measures to limit both legal and illegal immigration.

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