President Donald Trump's lawyer says the criminal investigation into possible collusion with Russia in last year's election could be over by December, but special counsel Robert Mueller's probe is expected to continue well into next year, according to a U.S. official.

Mueller continues to gather evidence and pursue investigative leads, as shown by steps like a subpoena he sent to more than a dozen Trump campaign operatives in October, according to the official with knowledge of the investigation, who requested anonymity.

Ty Cobb, the top White House lawyer handling the probe, has been consistently optimistic about Mueller's probe and its likely outcome, predicting the investigative cloud hanging over Trump and the White House should clear by early next year.

But the official with knowledge of the investigation, as well as outside legal experts, made clear that months of work still lie ahead for Mueller. For one thing, Mueller indicted Trump's former campaign chairman, Paul Manafort, last month, as well as another campaign aide, Rick Gates, for money laundering and other crimes. Manafort and Gates have said they aren't guilty, and Mueller's litigation against them is expected to continue well into 2018, the official said.

Mueller was given a broad mandate when he was appointed by the Justice Department in May to investigate whether Trump or any of his associates colluded with Russia as well as any other matters arising from that inquiry.

To build his case, Mueller has had to pursue multiple investigate angles beyond the White House, a second U.S. official said. Those include potential obstruction of justice related to Trump's firing of FBI Director James Comey and financial dealings in the U.S. and abroad by Trump family members and associates.