WASHINGTON — Congressional investigators are heading into 2018 with no immediate end in sight to their probes into Russia's meddling in the 2016 presidential election and possible collusion with the Trump campaign.

Every time the Senate Intelligence Committee interviews a witness, said its chairman Richard Burr, R-N.C., it learns the names of more people to question. The committee has interviewed more than 100 people since its investigation began in January.

"Every individual that is added, it puts about three more weeks into an investigation, so that’s why it makes it difficult for me to look out," Burr said at the Council on Foreign Relations this month, adding that revelations in news stories have also contributed to a long witness list.

Rep. Adam Schiff of California, the senior Democrat on the House Intelligence Committee, told USA TODAY that he believes the House panel still has "many months of work ahead of us." He added, however, that he's worried that Republicans on the committee may be trying to rush the investigation to a "premature" close because of pressure from the White House to bring it to an end.

"There are still dozens of witnesses that need to be brought in," Schiff said. "There are a lot of interactions between the Trump campaign and Russia that still need to be fleshed out."

Schiff said the committee may seek access to former Trump campaign chairman Paul Manafort and campaign aide Rick Gates after their trials, which aren't expected to happen until May at the earliest.

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Manafort and Gates pleaded not guilty on Oct. 30 to charges of money laundering and conspiracy after being indicted by a federal grand jury as part of special counsel Robert Mueller's wide-ranging investigation into Russia's interference in the election.

Mueller's inquiry appears to be gaining steam. It has also produced guilty pleas and pledges of cooperation from ex-campaign adviser George Papadopoulos and former national security adviser Michael Flynn for lying to the FBI.

White House special counsel Ty Cobb has expressed optimism that Mueller's investigation is winding down, indicating earlier this month that all interviews that investigators requested of White House staffers have been completed.

Yet some analysts cautioned that there are no obvious signs of a finish line in sight.

"That may be the view from Mr. Cobb's eyes, but I would be shocked if (Mueller) was done with the White House inner circle," said Ron Hosko, a former assistant FBI director who worked for Mueller when Mueller was FBI director. "All of these indictments, guilty pleas and interviews could be just round one."

It is more likely, Hosko said, that some staffers and others in Trump's orbit would be called back for additional interviews, especially if the information provided in initial discussions did not support evidence already gathered.

Mueller's criminal probe on behalf of the Department of Justice is separate from the investigations being conducted by three different congressional committees. In addition to the two Intelligence Committees, the Senate Judiciary Committee has been running a more limited probe.

Mueller's investigation, including any resulting trials, is likely to go even beyond 2018, said Charles Tiefer, professor at the University of Baltimore School of Law and the special deputy chief counsel for the House Iran-Contra Committee's investigation of the Reagan administration.

"In contrast, I think the House Intelligence Committee may well shut itself down early in 2018," Tiefer said. He said Republican leaders of the committee, which has been plagued by partisanship during its investigation, appear to want to end the probe as soon as possible.

Schiff said there are "increasingly concerning signs" that the GOP majority may try to close down the investigation soon because of pressure from the White House. Meanwhile, Republicans on the committee have complained that Democrats want to drag out the probe to help their party try to win control of the House next year. Rep. Mike Conaway, R-Texas, who has been running the Russia probe for the House panel, declined to be interviewed for this story.

The Senate Intelligence Committee, which has conducted itself in a more bipartisan fashion, may agree to finish in late summer so that its report comes out well before the November 2018 congressional elections, Tiefer said.

"They want at least part of the report to have bipartisan support," the professor said. "That becomes harder to achieve when you get closer to an election."

Burr said it's possible that the Senate committee may come out with two reports: one during the first few months of 2018 to provide states with recommendations on ways to protect their election systems from Russian hacking, and a second later in the year that provides overall findings on Russia's election interference and any collusion between the Trump campaign and the Kremlin.

"I will answer you the same way I answered the president," Burr said when asked at the Council on Foreign Relations when the investigation would end. "When we have interviewed everybody that needs to be interviewed, and we feel like we have answered every question that the committee jurisdictionally should, we will finish."

Schiff said he believes the House Intelligence Committee has already learned a great deal from its investigation, which began in March.

The panel, he said, has been able to substantiate the findings of the U.S. intelligence community, which issued a report in January concluding that Russian President Vladimir Putin ordered an influence campaign designed to undermine public confidence in the 2016 U.S. election, hurt Democratic candidate Hillary Clinton and help Donald Trump. The report said there was no evidence of actual vote tampering.

"In terms of the broad objective of confirming the veracity of the report, I think we've done that," Schiff said.

The congressman said he also believes the committee has made "substantial progress" in documenting evidence of collusion between the Russians and the Trump campaign. He cited the Trump Tower meeting in June 2016 where Donald Trump Jr., Manafort and Trump son-in-law Jared Kushner met with a Kremlin-linked attorney and others in an effort to get "dirt" on Clinton.

"I do want to point out that it's one thing to say there is evidence of collusion and another to say that it can be proved beyond a shadow of a doubt," said Schiff, a former prosecutor. "That will be Mueller's decision to make."

Mueller's team is also investigating possible obstruction related to the president's abrupt firing of former FBI Director James Comey, who was overseeing the Russia inquiry. In the days after the May dismissal, Trump said in an interview with NBC News that he fired Comey because of his handling of the Russia inquiry.

Comey's firing, coupled with the recusal of Attorney General Jeff Sessions for his belated disclosure of contacts with Russian Ambassador Sergey Kislyak, prompted Mueller's appointment as special counsel in charge of the Russia investigation.

Schiff, meanwhile, said he also believes the intelligence committees were key in bringing out evidence that Kremlin-linked groups exploited American social media platforms with ads and other content aimed at sowing dissension in the U.S. before and after the election.

Facebook and other social media companies initially dismissed the idea that Russians used their platforms to influence the election in any serious way. However, under pressure from Congress, Facebook disclosed in September that an internal investigation had uncovered $100,000 in ads traced to a Russian "troll farm" that carried "divisive social and political messages" on topics such as gay rights, race, immigration and gun control.

"I think that was enormously important," Schiff said. "We had no idea of the extent or form it took. Now we see abundant, very graphic evidence that it happened.

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