Former Defense Secretary James Mattis told the Elizabeth Holmes jury that 'there came a point when I didn't know what to believe'

• James Mattis took the stand Wednesday in Theranos founder Elizabeth Holmes' fraud trial.
• He said he invested $85,000 in Theranos and shared details about its military pilot project.
• The former defense secretary also said Holmes was his "sole" source of information about Theranos.

Former US Defense Secretary James Mattis has revealed how much he invested in the now-defunct blood-testing startup Theranos.

Mattis took the stand Wednesday in the fraud trial of Theranos founder Elizabeth Holmes, testifying that he invested nearly $85,000 in the company, an amount he said was significant for someone who had worked in government for 40 years, according to reporting by the Wall Street Journal.

Mattis also recalled the moment Holmes asked him to join Theranos' board. "I cautioned her I was not a medical person," he said, noting that Holmes said she valued a variety of backgrounds among board members.

Mattis ultimately joined the board after leaving active duty in 2013. He said he received $150,000 a year to serve on the board. He testified that he invested and joined the board to "have skin in the game."

The retired four-star general also said Holmes was his "sole" source of information about the company. During his testimony, several emails between Mattis and Holmes were shown as evidence. They showed the two discussing plans to test out Theranos' technology for potential use in the military.

In an email in late 2011, Mattis told Holmes, "I'm trying to find a way to employ your device on a swift 'pilot project' or 'proof of principle' to expedite its entry to our forces."

Holmes later responded, "It is on us to drive forward to get a pilot. Will do all it takes to make this happen and work though this process."

Mattis went on to say that he wasn't aware when he joined the board that some of Theranos' tests were being run on third-party machines rather than the company's own Edison machines. After the Wall Street Journal published a bombshell investigation about the limits of Theranos' tests, Mattis began to question "whether or not Edison worked."

"There came a point when I didn't know what to believe about Theranos anymore," he testified, according to the Journal. "Looking back now I'm disappointed at the level of transparency...I couldn't see why we were being surprised by such fundamental issues."

Mattis is one of more than 200 people listed as potential witnesses in the trial.