The chief technology officer of Google’s autonomous car project has announced that he is leaving the company as it transitions from research into product development.

Chris Urmson, a key contributor to Google’s self-driving car effort since it began in 2009, said in an online post that he is “ready for a fresh challenge.” Under his direction, Google vehicles were subjected to 1.8 million miles of testing.

The move follows the appointment of automotive industry veteran John Krafcik, who was hired by Google to assume leadership of its car project in 2015. Krafcik lauded Urmson as an “incredible colleague & leader” via Twitter on Friday.

While Urmson made assurances in his departing post that “the mission is in capable hands,” the New York Times claims that Urmson was discontented with the project’s direction and had a falling-out with Google co-founder Larry Page.