Neuralink, a small and secretive brain computing startup founded by Elon Musk, has raised $39 million of a planned $51 million funding round, according to a filing Friday with the Securities and Exchange Commission.

It was not immediately clear if the funders included traditional venture capital investors or if they comprised Musk and Neuralink employees. A spokesman did not immediately respond to a request for comment. Two years ago, the company said in a filing that it had raised $27 million of a planned funding round of as much as $100 million.

Neuralink was registered as a California medical research company in 2016 and has operated quietly from San Francisco since then. The company has hired away several high-profile neuroscientists from various universities and contracted with a laboratory at the University of California, Davis to conduct research on primates.

The venture has both awed and repelled various Musk watchers over the possibilities that it seemingly offers for connecting brains to computers. In March, a paper by a group of researchers affiliated with Neuralink was posted online. It outlined a novel technique for implanting electrodes into rat brains.

In April, when asked by a Twitter user for a Neuralink update, Musk replied “Probably something notable to announce in a few months.”