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Elon Musk, the Tesla and SpaceX CEO and world's richest man, welcomed twins last year with an executive at one of his other companies, Neuralink, Business Insider  on Wednesday.

Musk, who posted a  on May 24 saying "USA birth rate has been below min sustainable levels for ~50 years" and pinned it to the top of his more than 100 million-follower Twitter account, quietly fathered the children with Shivon Zilis, who works for Musk at the company which hopes to develop an implantable computer chip for the human brain, according to documents obtained by Business Insider.

Keep scrolling for photos of Elon Musk through the years

The outlet obtained court filings pertaining to changing the children's legal names to incorporate the Musk last name, and Zilis' as part of the middle names. CNN Business could not independently confirm the contents of the documents, but a Travis County, Texas court docket obtained by CNN Business indicated that the name change petition was initially filed in April 2022 and granted in early May — matching the dates on the documents published by Business Insider. Those documents contain the court's stamp as well as Musk's signature, listing him as the father and Zilis as the mother.

Musk did not immediately respond to requests for comment sent to representatives at Tesla, SpaceX and Neuralink.

Zilis' professional ties to Musk date back to at least April 2016, according to her , when she became an advisor to OpenAI. Musk is one of the  of OpenAI, a nonprofit research laboratory with a stated mission to "ensure that artificial general intelligence benefits all of humanity."

In August 2020, Musk held a  intended to showcase the progress of Neuralink's technology, which it had implanted in a pig. Nearly a year later, Neuralink  could play Pong after the company's chips were inserted into their brains. But, more recently, Neuralink has had to address concerns about its testing practices, denying allegations of animal cruelty while confirming earlier this year that monkeys have died as part of the testing.

According to Zilis' LinkedIn, she worked for both Tesla and Neuralink beginning in May 2017; and currently works as a director of operations and special projects at Neuralink. In 2020, Zilis became a board member at OpenAI. Prior to working at Musk-helmed companies, Zilis worked for a Bloomberg venture capital fund (which  on the Forbes 30 under 30 list for venture capital in 2015), as well as IBM. She's also served on the boards of at least two other artificial intelligence-related organizations. Zilis did not immediately respond to a request for comment.

This is the second set of twins Musk is currently believed to have fathered. Musk and his first wife, Justine, had twins in 2004 before welcoming triplets two years later. (The two, who were married from 2000 to 2008, lost their first child; Justine, an author, famously wrote about what it was like to be married to, and divorce, Musk for  in 2010.) Musk subsequently had two children with musician Claire Boucher, who is better known as Grimes. Their  in December 2021 through a surrogate.