(CNN) Washington lobbyist W. Samuel Patten, who has been one of the most low-profile but potentially significant cooperators in the special counsel's office investigation, appears to still be involved with sensitive aspects of Robert Mueller and the Justice Department's work.

In a court filing Monday meant to update a judge on his case and whether he should proceed to sentencing, prosecutors revealed nothing. Instead, they filed the entire status update under seal, giving no public reason for keeping details of his case private.

The secret court filing Monday comes in contrast to several disclosures prosecutors previously made about Patten's admitted crimes — especially related to him procuring Trump inauguration tickets for a Ukrainian client — and the related lobbying work for Ukrainians done by former Trump campaign chairman Paul Manafort.

Patten previously admitted to procuring Trump inauguration tickets illegally for a Ukrainian oligarch and a Russian closely associated with Paul Manafort. Patten was a corporate lobbyist in business with the former Trump campaign chairman's longtime Russian associate Konstantin Kilimnik, whom the Mueller team has asked about in recent months and accused of having ties to the Russian intelligence group that allegedly hacked the Democrats in 2016.

Patten agreed to cooperate with the Mueller investigation and other Justice Department actions before Manafort pleaded guilt y to criminal charges in September. Manafort had been Mueller's target for almost a year before his plea deal — and the Mueller team initially charged him with a host of financial crimes and foreign lobbying violations. A jury found Manafort guilty of tax and bank fraud related to his Ukrainian lobbying proceeds, then Manafort flipped and agreed to help prosecutors in September to avoid a second trial related to his foreign lobbying operation.

Read More