Kittlaus, sources told All Things D, had been planning a departure from Apple for a while due to a desire to take time off, be closer to family and brainstorm new ideas.
He and his co-founders Adam Cheyer and Tom Gruber spun Siri out of a project by non-profit R&D institute SRI International in 2008. SRI International had been asked by the Department of Defense's innovation arm, DARPA, to create an "enduring personalized cognitive assistant," which inspired the consumer-branded product. Apple acquired the company in April 2010.
Since the acquisition, Kittlaus has led Siri and speech recognition teams at Apple.
Kittlaus left just after the launch of the iPhone 4S on "amicable" terms, according to All Things D. Both other co-founders remain at Apple, according to their LinkedIn profiles.