Fresh reporting raises new questions about President Donald Trump's possible ties to Russia and emphasizes the need to ensure that special counsel Robert Mueller's investigation reaches its conclusion without interference, Senate Democrats said Sunday.

The New York Times reported Friday that the FBI had opened a probe in 2017 to determine whether the president had worked, knowingly or unknowingly, on behalf of Russia and against U.S. national interests. On Saturday, a Washington Post story said Trump went to great lengths to hide details of his discussions with Russian President Vladimir Putin.

The reports build on previous questions about Trump's connections with Russia that need to be investigated, Democratic Sen. Tim Kaine of Virginia said on NBC's "Meet the Press." Mueller's probe into Russian interference in the 2016 presidential election must be allowed to finish to provide the answers, he said.