Apple Inc CEO Tim Cook is shown with TV personality James Corden and musician Pharrell during a taped comedy bit in this image shot from a projection screen during an Apple media event in San Francisco, California, U.S. September 7, 2016.

Apple CEO Tim Cook has repeatedly called autonomous systems the "mother of all AI projects," and the company has a car endeavor underway, reportedly dubbed Project Titan.

But on a conference call with investors on Tuesday, Cook hinted that Apple has a single, large project in the works, and a vehicle is just one possibility. Here's what he said after reporting third-quarter results:

"In terms of the autonomous systems, what we've said is we are very focused on autonomous systems from a core technology point of view. We do have a large project going and are making a big investment in this. From our point of view, autonomy is sort of the mother of all AI projects, and the autonomous systems can be used in a variety of ways, and a vehicle is only one. But there are many different areas of it, and I don't want to go any further with that."

As Cook noted, some of the main technologies in self-driving cars, like Lidar sensors, are also used in robots.

Cook's vagueness may just be to keep the car project under wraps. Sources familiar with the situation told CNBC in June that Apple is leasing six cars from a Hertz subsidiary for autonomous software testing.

— FactSet and CNBC's Josh Lipton contributed to this report