• Elon Musk wrote and then deleted a post on X that appeared to question why there weren't more assassination threats made against President Joe Biden and Vice President Kamala Harris.
• The post came hours after the second apparent assassination attempt of Donald Trump, at his West Palm Beach golf club on Sunday.
• Musk endorsed Trump in July, after the Republican presidential nominee survived his first assassination attempt at a Pennsylvania rally.

Tesla CEO and X majority owner Elon Musk wrote and then deleted a Sunday post on social media platform X that appeared to question why there weren't more assassination threats made against President Joe Biden and Vice President Kamala Harris.

Musk, who has 197.8 million listed followers on X, posted the message shortly after a second apparent assassination attempt against Republican former President Donald Trump.

The post was prompted by an X user who asked, "Why they want to kill Donald Trump?"

Musk replied, "And no one is even trying to assassinate Biden/Kamala," punctuating his sentence with a thinking face emoji.

Both Biden and Harris have received assassination threats while in office.

Musk immediately faced backlash for the post, but he stood by it and defended it for roughly 9 hours before he deleted it.

Within an hour, Musk's tweet had been viewed by at least 1.3 million users, while more than 3,000 users had reposted it and at least 18,000 users had liked it.

Hours after the initial post was deleted, Musk penned two other X posts in which he claimed the original one was an ill-received joke.

"Well, one lesson I've learned is that just because I say something to a group and they laugh doesn't mean it's going to be all that hilarious as a post on X," Musk posted at 2:58 a.m. E.T. on Monday.

He followed up that post two minutes later: "Turns out that jokes are WAY less funny if people don't know the context and the delivery is plain text."

Musk has come under fire for controversial X posts before, but this latest episode was a rare instance of the billionaire deleting a post in response to criticism.

For example, in March 2021, the National Labor Relations Board ordered Tesla to have Musk delete a social media post that it saw as a threat to union organizers. As of Monday, the tweet remains posted on the site.

The Trump campaign did not immediately respond to CNBC's request for comment on Musk's post.

Spokespeople for X press relations did not reply to a request for comment from CNBC on Sunday, nor did Musk himself.

The White House denounced Musk's language in a statement Monday.

"As President Biden and Vice President Harris said after yesterday's disturbing news, 'there is no place for political violence or for any violence ever in our country,' and 'we all must do our part to ensure that this incident does not lead to more violence,'" White House spokesperson Andrew Bates said. "Violence should only be condemned, never encouraged or joked about. This rhetoric is irresponsible."

On Sunday, Trump was unharmed after what the FBI said appeared to have been an assassination attempt.

Shortly before 2 p.m. E.T., while Trump was playing golf at his West Palm Beach club, the former president was rushed to a safe location, moments after the Secret Service opened fire at a gunman with a rifle who was 300 yards to 500 yards away from Trump.

The suspect, Ryan Wesley Routh, made his first court appearance on Monday.

Musk, also the CEO of major aerospace and defense contractor SpaceX, publicly endorsed Trump in July, just hours after the Republican presidential nominee survived his first assassination attempt at a Pennsylvania rally.

Tom Nichols, a former professor at the U.S. Naval War College, raised the question of Musk's Pentagon contracts on Sunday, after Musk's post about Biden and Harris. "I had a security clearance for most of my adult life. If I had said something like this, I would've lost it instantly. And yet this guy is still a major government contractor," wrote Nichols.

Musk has emerged this election cycle as one of Trump's most visible allies, a stark reversal from their public feuding just two years ago.

Musk said he helped to create and fund a pro-Trump political action committee, America PAC.

In turn, Trump recently endorsed some of Musk's policy ideas, including a proposal to create a government efficiency commission to rein in federal spending. Musk has repeatedly volunteered to helm such a commission, which Trump has so far not ruled out.