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Peter Thiel is doubling down on his criticism of Google. The Silicon Valley investor wrote in a New York Times op-ed Thursday that the search giant is hurting the US by sharing its artificial intelligence technology with China.

Thiel, one of President Donald Trump's highest-profile supporters in the tech industry, said it's "shocking" that Google would work with a rival country, because AI is such a beneficial military tool.

He wrote that AI is "valuable to any army -- to gain an intelligence advantage, for example, or to penetrate defenses in the relatively new theater of cyberwarfare, where we are already living amid the equivalent of a multinational shooting war."

He also points to an AI lab Google opened in Beijing two years ago, as well as the company's decision to stop work with the Pentagon on Project Maven, an initiative that uses AI to improve the analysis of drone footage. Google decided not to renew its contract after its employees protested the project. Soon after, CEO Sundar Pichai released a set of guidelines that said Google wouldn't develop AI for weapons but would still work with the US military.

Google declined to comment on the op-ed. But in the past, the company has noted that it doesn't work with the Chinese military.

Thiel's comments come weeks after he accused Google of having a "seemingly treasonous" relationship with China and urged a probe by the FBI and CIA. After Thiel's comments, Trump tweeted that he would "take a look."

Last week, Treasury Secretary Steven Mnuchin said he met with Pichai to discuss the company's work in China. After the meeting, Mnuchin said he has no security concerns about Google's work in the country. Two days later, though, Trump tweeted that there "may or may not" be security concerns.