Uber CEO Travis Kalanick, left, with Anthony Levandowski. Associated Press Google and Uber started off as friends, then became competitors, and are now adversaries in a bitter legal fight to control the future of transportation.

In an explosive lawsuit filed Thursday, the Google self-driving-car group, now known as Waymo, accused Uber of using stolen technology to advance its own autonomous-car development.

The suit, filed in the US District Court in San Francisco, claims that a team of ex-Google engineers stole the company's design for the lidar laser sensor that allows self-driving cars to map the environment around them.

"Misappropriating this technology is akin to stealing a secret recipe from a beverage company," Waymo wrote in a blog post announcing the suit, which names both Uber and Otto, a startup Uber acquired, as defendants.

"Otto and Uber have taken Waymo's intellectual property so that they could avoid incurring the risk, time, and expense of independently developing their own technology," the 28-page lawsuit says. "Ultimately, this calculated theft reportedly netted Otto employees over half a billion dollars and allowed Uber to revive a stalled program, all at Waymo's expense."

The legal complaint marks the latest escalation in the increasingly antagonistic relationship between the two tech giants and reflects the big stakes involved in self-driving-car technology, which threatens to upend the $70 billion US automotive industry.

"We take the allegations made against Otto and Uber employees seriously and we will review this matter carefully," Uber wrote in a statement to Business Insider.

A suspicious email

At the center of the suit is Anthony Levandowski, one of the original members of the team that worked on Google's self-driving-car project. In January 2016, Levandowski left Google after nine years to found Otto, a startup focused on autonomous trucks. Six months later, Uber acquired Otto in a deal valued at $680 million.

The lidar sensor on top of an Uber car. Business Insider/Danielle Muoio Waymo alleges that Levandowski "downloaded over 14,000 highly confidential and proprietary design files for Waymo's various hardware systems, including designs of Waymo's LiDAR and circuit board," six weeks before resigning from Google.

Levandowski installed specialized software on his company laptop to gain access to Waymo's design server, Waymo alleges in the suit. He then downloaded 9.7 GB of highly confidential files and trade secrets that included blueprints, design files, and testing documentation, the suit alleges.

Waymo said in the suit that it had learned of the theft after it was inadvertently copied on an email that included machine drawings of what appeared to be Uber's lidar circuit board that "bears a striking resemblance" to Waymo's designs.

Waymo also claims that former employees now working at Uber and Otto downloaded "additional highly confidential information" related to its lidar system, including supplier lists, manufacturing details, and statements of work with highly technical information.

Google's self-driving-car prototypes for years used third-party laser sensors made by Velodyne. But the company announced in early January that it was building its own lidar system in-house, which allowed it to reduce the price of the notoriously expensive system by 90%.

A bumpy history

The lawsuit marks the latest escalation in the bumpy relationship between the two tech giants.

Google Ventures invested $250 million in Uber in 2013, when the ride-hailing service was still in its early years.

But as the two companies' business interests began to overlap, particularly around self-driving cars, the relationship began to fray. In August, Google executive David Drummond stepped down from his seat on Uber's board.

Before stepping down, Drummond, as well as Google Ventures CEO David Krane, had been shut out of Uber's board meetings.

Uber has become one of the most valuable private technology companies, with a valuation of nearly $70 billion. It has faced numerous challenges in its quest to build self-driving cars, notably in December when San Francisco shut down trials of the vehicles in the city because Uber had failed to obtain the proper permits.

Uber recently began testing the cars in Arizona instead.