Rick Gates, former campaign aide to U.S. President Donald Trump, departs after a bond hearing at U.S. District Court in Washington, U.S., November 6, 2017.

The judge at former Trump campaign chief Paul Manafort's trial has sealed a discussion with lawyers that ensued after Manafort's deputy Rick Gates was asked by a defense attorney whether special counsel Robert Mueller's team had questioned him about his campaign work.

Judge T.S. Ellis, in his sealing order signed Thursday, wrote that allowing the discussion to become public "would reveal substantive evidence pertaining to an ongoing government investigation."

While Manafort is on trial in U.S. District Court in Alexandria, Virginia, for alleged crimes that are unrelated to the 2016 presidential election, Mueller is continuing to investigate Trump campaign officials for possible collusion with Russians who were attacking the election. President Donald Trump has repeatedly called that probe a "witch hunt."

In a new court filing on Friday in federal court in Washington, Mueller and Gates jointly informed another judge that Gates continues to meet with Mueller's team as required by his plea agreement, and "the investigation, which includes the possible continued need for assistance from [Gates] ... is ongoing."

The sealed discussion occurred Tuesday in Ellis' courtroom in Alexandria as Gates was testifying against Manafort, a longtime Republican lobbyist and consultant.

Gates earlier this year pleaded guilty in the same case to conspiracy and to making false statements, and agreed to cooperate with Mueller in the prosecution of his former boss Manafort.