Former US secretary of state George Shultz’s support for Elizabeth Holmes and her fraudulent blood testing company, Theranos, which devastated his family and caused a bitter feud with his grandson, receives fresh scrutiny in a biography published on Tuesday.

Shultz was Ronald Reagan’s top diplomat at the end of the cold war. Before that, he was secretary of the treasury and secretary of labor under Richard Nixon. He is now the subject of In the Nation’s Service, written by Philip Taubman, a former New York Times reporter.

Shultz joined the Theranos board of directors in 2011.

Taubman recounts how Shultz – then in his 90s and with no biomedical expertise – was impressed by Holmes’s startup and its promise to revolutionise blood testing. He helped the young entrepreneur form a board of directors and raise money from heavyweight investors including Rupert Murdoch.

“Shultz repeatedly told friends that Holmes was brilliant,” Taubman writes. “Over time, his associates grew alarmed, fearing that his enthusiasm was colored by personal affection for Holmes. He talked by phone with her almost every day and invited her to join Shultz family Christmas dinners. She encouraged his attention by leaning in close to him when they were seated together on sofas.”

Dismissing scepticism regarding Holmes’s claim to have come up with a quick and easy blood test that would dramatically simplify healthcare, Shultz encouraged his grandson, Tyler Shultz, to work a summer internship at Theranos and become a full-time employee.

But Tyler Shultz came to suspect that Holmes was overselling her technology and took his concerns to the Wall Street Journal. Suspecting the younger Shultz was the whistleblower, Holmes set her lawyers on him and put him under surveillance. Alarmed, Tyler Shultz went to his grandfather for help.

Taubman writes: “Instead of hugging his grandson and disowning Holmes, Shultz equivocated. He tried unsuccessfully to mediate between Tyler and Holmes.”

When that effort failed, Shultz refused to cut ties with the businesswoman. He told Tyler: “I’m over 90 years old. I’ve seen a lot in my time, I’ve been right almost every time and I know I’m right about this.”

Tyler felt betrayed. In a 2020 podcast, Thicker Than Water, he imagined three reasons why his grandfather sided with Holmes.

“One is that you were corrupt and have invested so much money in Theranos that you were willing to make ethical compromises in order to see return on your investment. The second is that you are in love with Elizabeth.

“So no matter how many times she lies to you, no matter how many patients she injures and no matter how badly she harms your family, you will put her above everything else. The last possibility is that you have completely lost your mental edge and despite an abundance of data showing that she was a criminal, you somehow are incapable of connecting these very, very big dots.”

Taubman also suggests motives: financial gain, as Shultz’s holdings in Theranos stock soared before Holmes fell to disgrace, peaking at $50m; or personal loyalty to Holmes, just as Shultz showed to Richard Nixon during the Watergate crisis and Reagan during the Iran-contra affair.

The author writes: “Shultz’s performance left his family broken. Saddened friends and associates attributed the conduct to his advanced age.”

In 2018, Holmes was indicted on charges involving defrauding investors and deceiving patients and doctors. Last year, she was sentenced to more than 11 years in prison, made a symbol of Silicon Valley ambition that veered into deceit.

Shultz sought to heal the rift with his grandson, stating that he had “made me proud” and shown “great moral character”. Tyler Shultz said his grandfather never apologised but their relationship “started to heal”. Taubman notes that the Holmes issue “remained unfinished business” when Shultz died in 2021, at the age of 100.

The biography was written over 10 years and draws on exclusive access to Shultz’s papers. It explores his involvement in the summits between Reagan and the Soviet leader Mikhail Gorbachev that ended the cold war, the Iran-contra affair and Internal Revenue Service investigations into Nixon’s “enemies”.