Alphabet’s (goog) Waymo self-driving car unit asked a U.S. judge on Monday to postpone an upcoming trade secrets trial against Uber Technologies (uber), so Waymo could investigate whether Uber withheld important evidence in the case.
The trial is currently scheduled to begin on Dec. 4 in San Francisco federal court. Waymo said it learned of new evidence last week after the U.S. Department of Justice shared it with the judge overseeing the case.
The two companies are battling to dominate the fast-growing field of self-driving cars.
In its court filing on Monday, Waymo said it recently learned that a former Uber security analyst sent a letter to an Uber in-house lawyer more than six months ago, which contained important facts about the case.
Waymo’s court filing is partially redacted from public view, so the details of the analyst’s letter are unclear. However, Waymo said Uber concealed the letter despite demands from Waymo and the judge to disclose all relevant evidence.
Representatives for Uber could not immediately be reached for comment.
Waymo sued Uber in February, claiming that former Waymo executive Anthony Levandowski downloaded more than 14,000 confidential files before leaving to set up a self-driving truck company, called Otto, which Uber acquired soon after.
Uber denied using any of Waymo’s trade secrets. Levandowski has declined to answer questions about the allegations, citing constitutional protections against self-incrimination.
For more about the Waymo-Uber lawsuit, watch Fortune’s video:
Earlier this year U.S. District Judge William Alsup, who is hearing the civil action brought by Waymo, asked federal prosecutors to investigate whether criminal theft of trade secrets had occurred. That probe is being handled by the intellectual property unit of the Northern California U.S. Attorney’s office, sources familiar with the situation said. No charges have been filed.
Alsup disclosed last week that he had received a letter from prosecutors, which he did not reveal. However, Alsup ordered the former Uber security analyst, the Uber in-house lawyer and another witness to appear in court on Tuesday at a final pretrial conference.
It is unusual for prosecutors to share information with a judge days before a civil case is set to begin.
Alsup already delayed the trial once before, in October, citing Waymo’s need to probe separate evidence Uber had not promptly disclosed.