Artificial intelligence is a growing field for tech companies, but among futurists and other tech industry figures, the potential unintended consequences of AI are an equally growing concern. Count Elon Musk among those worried about AI’s possible effects.

At the National Governors Association’s 2017 meeting, Musk warned about the disruptive effects in robotics and other fields, cautioning that people “should be really concerned about AI.”

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“Until people see robots going down the street killing people, they don’t know how to react because it seems so ethereal,” Musk said. “AI is a rare case where I think we need to be proactive in regulation instead of reactive. Because I think, by the time we are reactive in AI regulation, it’s too late.”

Musk also expressed support for ways to proactively regulate the AI industry.

“The way regulations are set up is that a whole bunch of bad things happen, there’s a public outcry, and after many years a regulatory agency is set up to regulate that industry,” Musk said. “It takes forever. That, in the past, has been bad but not something which represented a fundamental risk to the existence of civilization. AI is a fundamental risk to the existence of human civilization.”

Among Musk and others, they have argued that the technology has the potential to be a long-term threat for humanity, due to the possibility that AI that could reach the point where it could become smarter and faster than human intelligence. These concerns have also not been limited to possible future scenarios for AI. In the immediate future, Earlier this year, Sun Microsystems co-founder Vinod Khosla said AI could become smart and fast enough to make medical specialists like oncologists and radiologists obsolete.

These concerns come amid a major upswing in AI investment and research among tech companies. Companies including Google and Microsoft have made significant pushes into AI research and venture capital firms have broadened their focus towards AI investment.

Publicly, Musk’s hesitancy towards AI has not been a new concern. The Tesla CEO has frequently discussed his unease towards the technology in the past.

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Elsewhere in Musk’s talk, he also focused on areas including energy investment and the prospect of self-driving cars becoming a normal part of users’ lives. Check out the full talk below and Musk joins at around the 26:30 mark: