Buried in a criminal complaint against a former Apple engineer who's being charged with stealing trade secrets is a remarkable revelation about the size of Apple's autonomous driving systems project: 5,000 employees are working on it or know about it.

Xiaolang Zhang, who Apple hired to work on software and hardware for self-driving cars in December 2015, is accused of downloading files with proprietary information during paternity leave, according to a criminal complaint filed in federal court in Santa Clara, California, on Monday.

Zhang told Apple he was leaving the company to join X-Motors, based in Guangzhou, China, after returning from his paternity leave. During his break, security footage showed Zhang returning to campus past 9 p.m. on a Saturday, against corporate policy, and leaving with a computer keyboard, some cables and a large box, according to the suit.

According to the complaint, about 5,000 of Apple's 135,000 employees (3.7 percent) are "disclosed on the Project," which Apple has never openly discussed. Of those employees, 2,700 are designated as "core employees" on the project, giving them access to certain databases. The term disclosed refers to people working on or knowledgeable of the company's efforts in autonomous driving and related technology.

"Although Apple has made general statements to the press about being interested in autonomous vehicle development, the details of Apple's research and development for the Project is a closely guarded secret that has never been publicly revealed," the complaint says.

Zhang's role at Apple was designing and testing circuit boards to analyze sensor data, and he admitted to both Apple and the FBI that he took the company's data and airdropped it to a personal computer, according to the complaint.

Reports of Apple's self-driving car efforts started surfacing several years ago. Apple has obtained permits to test self-driving cars in California and recently hired an executive from Waymo, Alphabet's self-driving car unit.

"Apple takes confidentiality and the protection of our intellectual property very seriously," the company said in a statement. "We’re working with authorities on this matter and will do everything possible to make sure this individual and any other individuals involved are held accountable for their actions."

Zhang faces up to 10 years imprisonment and a $250,000 fine, according to the complaint.

You can read the full complaint here.

Correction: A previous version of the story mistakenly said that 5,000 Apple employees are working on autonomous driving technology. The complaint says 5,000 people are "disclosed on the project," which includes employees working on it or familiar with it.