Waymo, the self-driving unit of Google parent company Alphabet, clashed with Uber in a critical legal battle that threatens to halt the ride-hailing service's self-driving car program. Waymo is suing Uber for allegedly using autonomous car technology stolen by former Waymo employee Anthony Levandowski.

Waymo has also urged the judge to stop Uber's self-driving program while legal proceedings are ongoing. But the judge said Waymo does not "have a smoking gun."

At a court hearing in San Francisco on Wednesday, Waymo's lawyers said Levandowski downloaded and stole about 14,000 documents shortly before he left the company in January 2016.

Waymo also alleged that Levandowski started his independent self-driving truck company Otto as part of a "cover-up scheme" so Uber could eventually acquire it and use Waymo's intellectual property to develop a LiDAR (Light Detection and Ranging) system for its own program. Otto was acquired by Uber for $680m in August 2016.

"Through discovery we've learned that Uber and Levandowski created a coverup scheme for what they were doing," Waymo's lawyer Charles Verhoeven said. "They concocted a story for public consumption. The story was that Mr. Levandowski left Waymo for his own company."

Waymo's legal team pointed out that Uber gave Levandowski $5.3m shares of Uber stock the day after he quit Google "without giving notice." This amount of stock in Uber was worth around $250m at the time.

Uber later said the stock was given to Levandowski after he was hired as head of Uber's self-driving program. It added that it backdated the vesting start date to 28 January to account for his time at Otto as well.

Waymo alleges that Uber used details about its proprietary LiDAR technology, which allows self-driving cars to "see" what's around them and navigate, to develop their own . Uber, however, argues that its technology is "fundamentally different" from that of Waymo's.

Levandowski pleaded the 5th Amendment which protects him against self-incrimination during questioning.

Uber denied that it stole Waymo's technology or used its trade secrets. The company also maintained that there is no evidence to suggest that Uber knew about the 14,000 downloaded files before the lawsuit was filed.

"We've interviewed more than 85 Uber employees, and more than 40 attorneys spent more than 6,000 hours reviewing documents, including over weekends," Uber lawyer Arturo Gonzalez said. "After reviewing more than 300,000 documents, we've only found one Google email in the files.

"We're not hiding anything your honor. There is no evidence that anyone at Uber knew anything about these 14,000 files before this lawsuit was filed."

Waymo's lawyers accused its former employee Anthony Levandowski and Uber of creating a cover-up firm to steal its self-driving car technology. Reuters/Aaron Josefczyk/File Photo

US District Judge William Alsup said that all that has been proven is that Levandowski downloaded the 14,000 files. However, he was not convinced whether or not Uber actually used any trade secrets from Waymo.

"I've given you lots of discovery and so far you don't have a smoking gun," judge Alsup said.

While Uber argued that the allegedly stolen files never touched its servers, the judge said that it did not prove that the company did not use the allegedly stolen information. He laid out a hypothetical case that Levandowski may have brought his personal laptop into Uber's offices and worked off of that.

"Nothing prevented that from happening", Uber's attorney said, adding there is no evidence that it did.

Meanwhile, Uber said last week that Levandowski would be stepping down as head of Uber's self-driving program during the case proceedings.

The judge did not rule on the injunction by the end of the day, but is expected to do so in a few days. He added that he is still considering whether to send Waymo's trade secret claims to arbitration.