Apple NASDAQ:AAPL has reportedly sent legal letters to about 40 former employees now working at OpenAI as its trade-secrets lawsuit against the AI startup gathers pace.

According to the Financial Times, the letters ask the former employees to preserve documents and communications tied to Apple's allegations and to meet with its lawyers. As many as 400 former Apple employees are now believed to work at OpenAI.

Apple makes the iPhone, Mac and other consumer devices, while OpenAI develops AI models and is preparing to move more directly into hardware. That is where the dispute becomes more important.

Apple alleges OpenAI used current and former employees to obtain confidential hardware designs and details about unreleased technologies, products and manufacturing processes. It also claims the conduct was directed by OpenAI leadership.