(CNN) Special counsel Robert Mueller's office is defending an agreement it made with a Dutch lawyer tied to former Trump deputy campaign chairman Rick Gates to waive his right to request public records.

In a new court filing, prosecutors make clear that Alex van der Zwaan knows key details about Mueller's ongoing investigation.

"Van der Zwaan is in an unusual position of having information related to the office's investigation that is not widely known -- including information that he knows first-hand due to his role in the conduct the Office is investigating," the filing on Monday morning says.

Last week, prosecutors revealed that van der Zwaan was privy to fall 2016 communications between Gates and a person with ties to the Russian military intelligence service, and that his knowledge of those communications was relevant to the investigation.