Elon Musk has accused President Biden of causing something "far worse than 9/11."
The CEO of X, formerly Twitter, accused Biden of "treason" over allegations of "importing voters" ahead of this year's presidential election.
Musk has been an outspoken critic of President Biden's immigration policy.
It comes after a secret Biden programme was exposed where the administration admitted to flying 320,000 migrants into the US, but officials refuse to say who was flown and to where.
Elon Musk has slammed the President
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Taking to social media, Musk wrote: "Treason indeed! Ushering in vast numbers of illegals is why Secretary Mayorkas was impeached by the House.
"They are importing voters. This is why groups on the far left fight so hard to stop voter ID requirements, under the absurd guise of protecting the right to vote."
He later added: "This administration is both importing voters and creating a national security threat from unvetted illegal immigrants.
"It is highly probable that the groundwork is being laid for something far worse than 9/11. Just a matter of time."
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Tesla Chief Executive Officer Elon Musk ReutersMusk continued: "Three things America needs: - Secure borders - Safe cities - Sensible spending."
He also reposted a post from right wing commentator Mario Nawfal, who claimed that "US has five million more illegal migrants than it did in 2020.
"A diagram showing the exponential increase in illegal migration since Biden took office. The total number of illegal migrants in the country has almost tripled in that time."
He responded saying: "Wow"
Earlier this week, an appeals court temporarily set aside a judge's ruling that blocked a Texas law giving state officials broad powers to arrest, prosecute and order the removal of people who illegally cross the US Mexico border.
The order on Saturday from the New Orleans-based 5th US Circuit Court of Appeals means the law, known as SB4, could take effect as the administration of Democratic President Joe Biden pursues claims that it interferes with the federal government's enforcement of US immigration laws.
But the 5th Circuit said it would stay its decision for seven days to give the federal government a chance to appeal to the US Supreme Court.