Apple is advancing its self-driving tests, operating a 27-vehicle fleet in California.

According to a Bloomberg Technology report, the California Department of Motor Vehicles confirmed the registration of 24 Lexus RX450h SUVs, in an emailed response.

These vehicle registrations were staggered throughout the year, following three in April 2017. Of the vehicles, it is known the first three were leased from car rental giant Hertz, which roved around San Francisco, Calif. streets. No word if the remaining 24 are from Hertz as well.

Apple has been stunningly secretive over its self-driving projects, with CEO Tim Cook offering limited insights in the past year. In a June 2017 Worldwide Developers Conference Bloomberg interview, Cook acknowledged Apple’s efforts in building an autonomous system, rather than an Apple-branded car.

“We’re focusing on autonomous systems, and clearly one purpose of autonomous systems is self-driving cars,” said Cook at the time. “There are others, and we sort of see it as the mother of all AI projects. It’s probably one of the most difficult AI projects actually to work on.”

To date, no word has been given by Apple for the latest registrations nor an official statement from Apple. In Nov. 2017, two of Apple’s AI and machine learning researchers published a white paper titled “VoxelNet: End-to-End Learning for Point Cloud-Based 3D Object Detection.” The paper was provided to popular repository, and described a neural network capable of improved detection over traditional LIDAR systems. This white paper provided one of the company’s first clues into its future plans.

One month later, Apple filed a patent application for a navigation system that uses sensors to determine driving routes, rather than use pre-existing data, complete with a diagram. It would also be capable of making on-the-fly adjustments with re-routing intelligence in real-time, based on weather, traffic, and other environmental and infrastructure-related conditions.