But people familiar with the probe say that such assurances are unlikely and that the meeting could trigger a new, more contentious phase between the special counsel and a frustrated president, according to administration officials and advisers close to Trump.

People with knowledge of the investigation said it could last at least another year — pointing to ongoing cooperation from witnesses such as former Trump campaign adviser George Papadopoulos and former national security adviser Michael Flynn, as well as a possible trial of two former Trump campaign officials. The special counsel’s office has continued to request new documents related to the campaign, and members of Mueller’s team have told others they expect to be working through much of 2018, at a minimum.