In a letter released today, Multnomah County District Attorney Rod Underhill ruled that the city of Portland does not have to release the complete set of records that show how the ride-hailing company Uber used a software called "Greyball" to evade regulators.

Underhill ruled that the records are "trade secrets, unconditionally exempt from disclosure."

Underhill's letter comes in response to a public-records appeal filed by WW and Reuters.

Uber used Greyball in 2014 to help drivers illegally operate in Portland and evade regulators. The Greyball technology hid Uber cars from city inspectors who attempted to use the Uber app to hail a car.

As part of its investigation into Uber, Portland subpoenaed documents related to Greyball.

The city's subpoena included a request for documents "relating to any software or software application known as Greyball." In other words, the city was asking to look at the rule-breaking software and the manual for how to use it.

WW asked for all the records the city had received. The city declined to provide them, saying they were covered in a nondisclosure agreement the city had signed with Uber.

Today, Underhill sided with the city.

Underhill cited a 2016 Multnomah County Circuit Court decision that said information on "individual Uber rides constituted a trade secret."

"As to the others, we find that information could show a competitor the technical mechanisms used by Uber to run its operations constitutes a trade secret where, as here, substantial legal efforts have been made to ensure secrecy," he wrote.