WASHINGTON (Reuters) - The Senate Intelligence Committee probe of Russian meddling in the 2016 election is nowhere near over, as lawmakers probe issues including a June 2016 meeting between top aides to then-Republican candidate Donald Trump and a Russian lawyer, the panel’s top Democrat indicated on Friday.

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Senator Mark Warner said committee staff have interviewed everyone at the meeting, where President Trump’s son Donald Jr. expected to be given derogatory information about Democratic candidate Hillary Clinton, with the exception of “one or two individuals who are Russian.”

“But I feel very strongly that you can’t, you could never conclude without the senators themselves being able to talk to the principals involved,” Warner said. “We have not gotten there yet.”

Warner, in an interview with Reuters, said the Senate investigation has made progress on several fronts.

It has, he said, “re-validated” a Jan. 6, 2017, U.S. intelligence assessment that Russian President Vladimir Putin ordered an influence campaign aimed at the 2016 presidential election, with the goal of undermining Americans’ trust in their institutions and denigrating Clinton.

In an effort that has been “frustratingly slow,” Warner said, the investigation also prompted the U.S. Department of Homeland Security to warn 21 U.S. states whose election systems were the subject of tampering attempts by Moscow.

“DHS has upped its game. The state election officials have upped their game. And I think we are – not as fast as I’d like – but I think electoral systems are going to be on better guard” for congressional elections in November this year, Warner said.

Warner and fellow Democrats have worked closely with the Senate panel’s Republican chairman, Senator Richard Burr. The bipartisanship contrasts with a parallel investigation by the House Intelligence Committee, where inter-party feuds have imperiled the effort, and may lead committee Republicans and Democrats to issue competing reports.

In the interview, Warner repeated a warning that he made in a Dec. 20 Senate floor speech that any move by Trump aimed at firing Special Counsel Robert Mueller would provoke a “constitutional crisis.”

Mueller is conducting a criminal probe of whether the Trump campaign colluded with Russia, something the White House denies. Two Trump campaign associates, former national security adviser Michael Flynn and aide George Papadopoulos, have pleaded guilty to lying to the FBI in Mueller’s probe.

Warner said his concern that Trump might take a step to dismiss Mueller was confirmed by news reports on Thursday detailing steps the president reportedly took to blunt the Russia investigation.

Trump, the reports said, ordered White House counsel Donald McGahn to convince Attorney General Jeff Sessions not to recuse himself from the Justice Department’s Russia probe, so Sessions could remain in charge of it. Sessions recused himself anyway.

“If true, it’s one more example of this president’s unusual behavior,” the senator said.

“And it’s one of the reasons why I don’t take as absolute the White House’s assurances, ‘oh no, we have no plans’” to fire Mueller. “Because clearly this is based upon the president’s actions on a variety of topics – things can change on a dime.”