Rick Gates, who was the number two to President Donald Trump's former campaign chairman Paul Manafort, is still cooperating with the special counsel's investigation, a court filing said Wednesday, raising the possibility that Robert Mueller's probe could extend into 2019.

The filing, from both the special counsel team and an attorney for Gates, told a federal judge in Washington not to begin sentencing yet, as Gates "continues to cooperate with respect to several ongoing investigations."

The attorneys said they did not plan to submit another report to the judge until January 15.

Gates was a longtime associate of Manafort's and was Manafort's deputy during Trump's 2016 campaign.

Gates was among those initially charged last year by Mueller, and he pleaded guilty to two charges last February. His testimony against Manafort made up a substantial portion of the former campaign chairman's Virginia trial in August, where Manafort was found guilty on eight counts of financial crimes. Manafort himself pleaded guilty in September in a separate case and agreed to cooperate with the Mueller investigation.

Gates' attorney said as recently as last month that his client was continuing to cooperate with the investigation, and Wednesday's joint filing submitted by Mueller showed that cooperation was continuing still.