Elizabeth Holmes, the founder of the blood-testing startup Theranos, is the subject of a new HBO documentary called "The Inventor: Out for Blood in Silicon Valley."

The documentary, along with a nonfiction book and a podcast series, offers a fascinating look at Holmes, who successfully convinced investors that Theranos had working technology and accurate results when it did not.

Of special note is the mystery around Holmes' baritone speaking voice, which Theranos insiders have said was an affect, perhaps meant to help her fit in the male-dominated Silicon Valley tech scene.

Former Theranos employees have suggested that Holmes' voice was faked and recalled instances in which she apparently slipped and her voice would raise a few octaves.

By several accounts, Elizabeth Holmes carefully crafted and maintained a persona while heading up her blood-testing startup, Theranos — including adopting a distinctively deep voice that former employees suspect she fabricated altogether.

Holmes' quirks as a leader of the now-dissolved Theranos have been well-documented: She methodically designed a persona borrowing from Silicon Valley elite, such as her notable Steve Jobs-esque outfits of black turtlenecks and slacks.

But the release of "The Inventor: Out for Blood in Silicon Valley," an HBO documentary on the rise and fall of Theranos, has pushed the mystery of Holmes' voice back into the spotlight.

In making the documentary, HBO filmmakers were leaked more than 100 hours of video footage taken internally at Theranos, including intimate one-on-one interviews with Holmes, giving a new look at the embattled entrepreneur and her company. In the footage, her baritone is on full display.

Her voice is a trademark, as many would-be investors and Theranos employees noted in the book "Bad Blood: Secrets and Lies in a Silicon Valley Startup." In the book, many said they were taken aback when first meeting Holmes, thanks to her unexpectedly deep voice and wide, unblinking eyes.

Former Theranos employees, however, have shared their suspicions that Holmes' deep voice was simply a ruse.

An employee recalled an evening with Holmes during which she "lapsed into a more natural-sounding young woman's voice," the Wall Street Journal reporter John Carreyrou wrote in "Bad Blood," published in 2018. The employee noted that the pitch was "several octaves higher than her normal baritone."

The former employee speculates that Holmes depended her voice because she was trying so desperately to fit in and be taken seriously as a young woman in the male-dominated Silicon Valley scene.

Read more: A former Apple employee inspired Theranos CEO Elizabeth Holmes' change from 'frumpy accountant' to her signature Steve Jobs-style black turtleneck

A podcast from ABC News released later in 2018 added more fuel to the rumors that Holmes had faked her signature deep voice. In the podcast, called "The Dropout," former employees said Holmes would sometimes "fall out of character" and revert back to what they called her authentic voice. This happened especially frequently after she had been drinking, employees said on the podcast.

Evidence of Holmes using a different voice is harder to find. The Cut reports that videos in which apparent vocal missteps can be heard are often taken down from the internet rather quickly, usually within a day or two.

But the release of HBO's documentary has put viewer focus square on Holmes' voice. People on social media have expressed intense fascination with her voice. The documentary is able to show Holmes' full persona in a way that makes the mystery of her voice that much more intriguing.

Holmes has not publicly commented about her voice or acknowledged whether it's authentic. That means we can only guess why she would choose to change her voice, which would require her to dedicate herself to remembering the ruse every time she spoke.

Some say she was taking whatever steps she could to make headway in the male-dominated Silicon Valley. Others wonder whether she was trying to further herself as much as she could from upspeak and vocal fry, two terms used to critique and police women's higher voices.

But whether or not the Holmes' voice was authentic, she was successful in convincing Silicon Valley bigshots that she was the real deal. She was able to get high-powered men with deep pockets to invest in her blood-testing startup. By signing on successful venture capitalists like Tim Draper and political heavyweights like former Secretary of State Henry Kissinger, Holmes was able to raise more than $700 million in investment capital.

At its peak, Theranos was valued at $9 billion, and Holmes was hailed as the world's youngest female billionaire. Since then, her company has been shut down, and she's facing charges of fraud from the Securities and Exchange Commission and the Department of Justice.