Liberal Sen. Bernie Sanders, I-Vt., said the White House's surprise firing of FBI Director James Comey Tuesday indicates that President Trump is involved in a cover-up.

"Donald Trump's decision to fire FBI Director James Comey raises serious questions about what his administration is hiding," Sanders tweeted Tuesday evening.

The Vermont senator followed that with, "It is clear that whomever President Trump handpicks to lead the FBI will not be able to objectively carry out the Russia investigation."

He called for "an independent investigation into the Trump campaign's ties to Russia."

In January, Sanders told ABC's "This Week" that he would welcome a resignation by Comey. "I think he should take a hard look at what he has done. And I think it would not be a bad thing for the American people if he did step down."

Sanders said at the time that Comey was "not doing investigations about the possible ― possible ties between Trump's campaign and the Russians."