The U.S. Secret Service said on Monday it was aware of a post by billionaire Elon Musk on the X social media platform musing about an absence of assassination attempts on President Joe Biden and Vice President Kamala Harris.

Musk, who owns the platform, formerly known as Twitter, put up the post after a man suspected of planning to assassinate Republican former President Donald Trump at his golf course in West Palm Beach was arrested on Sunday.

A Trump supporter and the CEO of Tesla (TSLA.O), opens new tab, Musk wrote on Sunday: "And no one is even trying to assassinate Biden/Kamala," a post he ended with an emoji of a face with a raised eyebrow.

He was quickly criticized by X users from the left and right, who said they were concerned his words to nearly 200 million followers could incite violence against Biden and Harris.

Musk deleted the post but the Secret Service, tasked with protecting current and former presidents, vice presidents and other notable officials, took notice.

"The Secret Service is aware of the social media post made by Elon Musk and as a matter of practice, we do not comment on matters involving protective intelligence," a spokesperson told Reuters in an email. "We can say, however, that the Secret Service investigates all threats related to our protectees."

The spokesperson declined to specify whether the agency had reached out to Musk, who seemed to suggest in follow-up posts that he had been making a 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," he wrote. "Turns out that jokes are WAY less funny if people don't know the context and the delivery is plain text."

Harris, a Democrat running against Trump in the Nov. 5 presidential election, issued a statement on Sunday night as did Biden expressing relief and gratitude that Trump had not been harmed and condemning political violence.

The White House criticized Musk for his post.

"Violence should only be condemned, never encouraged or joked about. This rhetoric is irresponsible," White House spokesperson Andrew Bates said on Monday.