Technology giant Google came on a backfoot when Chris Urmson, the self-driving car project's Chief Technical Officer (CTO), announced his departure.
Apart from the CTO, two other key employees are reportedly also leaving, CNET.com reported on Saturday.
"After leading our cars through the human equivalent of 150 years of driving and helping our project make the leap from pure research to developing a product that we hope someday anyone will be able to use, I am ready for a fresh challenge," Urmson wrote, without giving specifics about his future plans.
The other key employees -- Jiajun Zhu and Dave Ferguson -- who worked on the software of the project, are also leaving.
Google parent Alphabet did not respond to a request for confirmation on these departures yet, but regarding Urmson, Alphabet said he was a key member and wished him well.
"Chris has been a vital force for the project, helping the team move from a research phase to a point where this life-saving technology will soon become a reality. He departs with our warmest wishes," the company was quoted as saying.
John Krafcik, CEO of the self-driving-car project, wished Urmson well in a tweet, saying, "Chris is an incredible colleague & leader. Thank you for your passion & humility. Good luck on your new adventures!"