Doug Field has rejoined Apple Inc. (AAPL), one month after leaving his position as the senior vice president of engineering at Tesla Inc. (TSLA).

In his new role, Field is set to be reunited with Bob Mansfield, the head of Apple’s secretive self-driving program Project Titan. The pair previously worked together on the iPhone maker’s line of Mac computers.

Blog Daring Fireball was first to report the news of Field’s return to Cupertino. An Apple spokesperson has since confirmed the hire.

Field joined Tesla nearly five years ago. Last year, he was put in charge of the electric automaker’s aggressive drive to ramp up production of the Model 3. However, by April, costly delays manufacturing the company’s four-door sedan led CEO Elon Musk to intervene and take over these responsibilities. Shortly after, Field took a leave of absence, before eventually departing Tesla altogether. (See also: Tesla Sheds More Executives, Holds Hackathon to Fix Bottlenecks.)

The engineer’s return to Apple has fueled further speculation about the iPhone maker's self-driving car project. Company executives have provided very little detail about Project Titan, although recent developments suggest that they are investing significant capital in the program.

Court documents filed last month said that as many as 5,000 people were given authorized access to information about Project Titan, according to Reuters. Meanwhile, Business Insider reported that an affidavit filed in an FBI investigation showed that Apple has been building prototypes and components, and had requirements for power, battery system, and "drivetrain suspension mounts” already in place. (See also: Apple: Theft Sheds Light on Self-Driving Car.)

Trading Places

Field’s decision to rejoin Apple represents another interesting twist in the battle for automotive talent between the iPhone maker and Tesla. In 2016, Apple hired Chris Porritt, Tesla’s former vice president of vehicle engineering. The following year, Tesla struck back, recruiting software developer Chris Lattner, one of the creators of Apple’s Swift programming language. Lattner left the company just six months later.

According to the Financial Times, Tesla's Musk once described Apple’s car project as the “Tesla graveyard.”