Special counsel Robert Mueller may continue with his investigation into President Donald Trump's alleged ties to Russia far longer than the White House has planned.
In conversations with friends and advisors, the president has said that the investigation would wrap up early in 2018, CNN reported in December. One of the president's lawyers, Ty Cobb, told The Washington Post in mid-November that the probe would likely finish shortly after the new year.
But experts and precedent suggest the investigation could last well into 2018, if not longer, meaning that one of the biggest clouds over Trump's nascent presidency and the Republican majority in Congress could continue to cast a pall as midterm elections approach.
Robert Ray, who was independent counsel in the Whitewater investigation into former President Bill Clinton's dealings, told CNBC that he thinks the Mueller probe could last "well into 2018, but hopefully concluded with major prosecutorial decisions reached before the midterm election cycle."
Harry Litman, a former U.S. attorney and deputy assistant attorney general, predicted that the investigation could last even longer. He cited the possibility that the special counsel could pursue a convoluted money trail that could extend the inquiry into 2019.
Mueller's team has reportedly sought information from Deutsche Bank about former Trump national security advisor Michael Flynn, while federal prosecutors in Brooklyn, New York, reportedly asked the German lender for information related to Trump aide and son-in-law Jared Kushner's family business. "That sort of paper case takes painstaking forensic work before you can bring witnesses in," Litman told CNBC.
Typical special counsel investigations take several years, although an analysis conducted by the news website FiveThirtyEight found that Mueller's is proceeding at a rapid clip compared with precedent.
The special counsel's office, which declined to comment for this article, has wrapped up its interviews with White House employees, but it may seek to do follow-up interviews.
Four people associated with the president, including former campaign chairman Paul Manafort and Flynn, have already been charged with crimes connected to the probe. Two of those, Flynn and Trump campaign foreign policy advisor George Papadopoulos, have pleaded guilty and are cooperating with investigators.
Flynn's attorney said in March that the retired lieutenant general "certainly has a story to tell," while Papadopoulos' fiancee recently told ABC News that the young advisor's tale "will make a big difference" in the investigation.
Flynn's cooperation, in particular, could indicate that Mueller is winding up one key part of his investigation. Sol Wisenberg, who was deputy independent counsel in the Whitewater investigation, said that Flynn's cooperation with investigators may allow Mueller to complete the portion of his probe of potential collusion between the Trump campaign and Russia "soon."
"Hard to believe that there would be criminal collusion without Flynn's knowledge," Wisenberg said. "Flynn did not plead to that and his plea deal doesn't seem to cover it."
Mueller's prosecution of Manafort and his business partner Rick Gates, who were both indicted in October on charges related to unlawful financial dealings, will continue regardless of the outcome of the collusion investigation.
A court date in that prosecution has been planned for May. A status report has been scheduled in the Flynn case for February.
Besides collusion, the special counsel is, according to The Washington Post, also pursuing an obstruction of justice investigation related to the president's firing of former FBI Director James Comey, among other possible matters.
The special counsel is authorized to investigate any matters that "may arise directly from the investigation," according to Deputy Attorney General Rod Rosenstein's order appointing the special counsel. There are no indications that Mueller has exhausted potential leads.
In a sign that the investigation could be pursuing new lines of inquiry, Yahoo reported Wednesday that Republican National Committee officials in recent weeks have started fielding questions from investigators related to the political organization's digital operations.
The investigation enters the new year under fire from the president, as well as subject to accusations of partisan bias from conservative commentators and lawmakers.
Republicans on the Hill were outraged after the release of text messages between two former Mueller prosecutors that showed the two disparaging the president, including calling him an "idiot."
Those two senior agents, Peter Strzok and Lisa Page, provided fodder for House Republicans who grilled Rosenstein at a House Judiciary Committee hearing Dec. 13.
House Judiciary Committee Chairman Bob Goodlatte, R-Va., called the revelation "deeply troubling to all citizens who expect a system of blind and equal justice."
Also in December, the president's transition organization challenged the seizure of its emails by the special counsel's office, calling the action "unlawful."
Trump, for his part, has lashed out against the FBI, saying its reputation is in "tatters."
To counter ire on the right, Democrats have sought to defend Mueller and his probe. In a Dec. 20 speech on the floor of the Senate, Virginia's Mark Warner – the ranking member of the Senate Intelligence Committee, which is conducting its own Russia probe – criticized what he called a "seemingly coordinated" line of attack on the Mueller investigation.
The White House has pushed back against rumors that Trump is planning to fire Mueller, however.
"For the thousandth time, we have no intentions of firing Bob Mueller," White House spokeswoman Sarah Sanders said on Fox News after Warner's Senate speech.
The White House did not respond to a request for comment from CNBC. The president has forcefully denied any misconduct in his campaign's contacts with Russia, and has called the investigation a "witch hunt."
By regulation, Rosenstein is the only person with the authority to fire Mueller or end the inquiry. Mueller is required to submit a status report in July 2018 regarding the progress of the investigation, at which point Rosenstein will determine whether the probe will continue and set its budget for the following fiscal year.
In addition to Mueller's investigation, at least three congressional investigations related to Trump's alleged ties to Russia are underway. Those investigations are unable to bring criminal charges and have largely been riven by partisan bickering.
"There are people who've already made up their minds waiting to see whether or not their previously held conviction will be validated," Rep. Trey Gowdy, R-S.C., told NBC News in December. "But I think most people are waiting on Mueller."