(NEW YORK) — Former Theranos board member and former Secretary of Defense James “Mad Dog” Mattis testified last week in the criminal trial of former Theranos CEO Elizabeth Holmes.

“I was taken with the idea that with one drop of blood and with remote capability, you could basically test for a broad array of problems,” Mattis told jurors from the witness stand during his initial examination from prosecutors.

When asked who was in charge in board meetings — Holmes or her former romantic partner and Theranos Chief Operating Officer Ramesh “Sunny” Balwani — Mattis did not hesitate when he said: “Ms. Holmes.”

And when asked where he got his information regarding Theronos blood testing devices, he also named Holmes. “I had no other source of information on it,” he told the court.

Holmes founded Theranos in 2003 and claimed the company was developing blood testing technology that could perform hundreds of blood tests using only a few drops of blood. Holmes and Balwani face a dozen charges of wire fraud and conspiracy to commit wire fraud in connection with what prosecutors call a multi-million dollar scheme to defraud investors and patients.

Both have pleaded not guilty. Balwani’s trial is slated to begin next year.

Along with Mattis, last week jurors heard from a digital forensic expert, Theranos’ former lab director, and the first Theranos patient to take the stand.

Mattis joined Theranos in 2013 and departed in 2016, prior to joining the Trump administration as the secretary of defense.

Mattis told jurors that when he joined the board in 2013 U.S. military casualties had not relented. “I was interested in anything that would improve the care of casualties,” he added.

Mattis recalled meeting Holmes in either 2011 or 2012, at an event in San Francisco where he was giving a speech. Holmes allegedly pricked his finger to give him “an idea of what the machine blood draw was,” he stated in court.

Mattis told the court that he became instantly impressed with Holmes’ description of Theranos’ testing capabilities, thinking the company’s devices could have game-changing benefits on the battlefield.

In 2013, he pushed for Theranos devices to join a “pilot project” where they’d be compared to devices already used on large U.S. Naval vessels, according to email exchanges between Mattis and Holmes displayed in court.

“We could do a side-by-side comparison,” Mattis recalled in the courtroom. The plan was to put Theranos devices up against the already-approved devices on those vessels to determine if the devices were “faster” or “more accurate.”

Mattis told jurors he invested $85,000 in the startup. His salary on the board was approximately $150,000, according to Theranos financial documents.

Theranos devices, however, never ended up in Mattis’ “pilot project” or in any sort of military arena. He testified that at a certain point, following growing scrutiny of the company’s testing capabilities, “I didn’t know what to believe about Theranos anymore.”

Since leaving the company’s board to join the Trump administration, Mattis has voiced regrets over his involvement with Holmes’ failed company. “The bottom line is we all make mistakes at times,” he told PBS in 2019.

Mattis may be the first of several notable names to testify in Holmes’ trial. Others who may be called as witnesses include former Secretary of State Henry Kissinger, billionaire media tycoon Rupert Murdoch and prominent attorney David Boies. Murdoch was a Theranos investor, and Boies at one point sat on Theranos’ board and his firm served as the company’s outside counsel for several years.

Another key witness from last week, Brittany Gould, was the first Theranos patient to take the stand. She told the court that the company’s blood test inaccurately showed she was suffering a miscarriage when in fact she had a healthy pregnancy.

Gould, who had already miscarried three times, testified that in September 2014 she took a Theranos blood test at a Walgreens store in Arizona after learning that she was pregnant.

Additional non-Theranos tests confirmed the initial results were inaccurate, and she went on to have a healthy baby girl.

Holmes’ lawyers elected not to cross examine Gould.

Holmes and her counsel did not respond to ABC News’ repeated requests for comment.

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