 -- President Donald Trump’s sudden dismissal of FBI Director James Comey on Tuesday came at a crucial moment in the investigation of Russian interference into the 2016 election, raising fears that the move could undermine the nearly yearlong probe.

“The inescapable conclusion from the circumstantial evidence here is the president wanted to stop or stifle this investigation,” Sen. Richard Blumenthal, D-Conn., told ABC News.

While the Department of Justice’s letter explaining Comey’s dismissal didn’t mention the Russia investigation, statements from the White House on Tuesday made clear that the White House believes the probe has run its course.

“When are they going to let that go?” deputy press secretary Sarah Huckabee Sanders said on Fox News. “It’s been going on for nearly a year. Frankly, it’s kind of getting absurd. There’s nothing there … It’s time to move on, and frankly, it’s time to focus on the things the American people care about.”

Officials told ABC News that the FBI investigation is broad and complex and is continuing apace. Agents are increasingly focused on Trump’s former national security adviser Mike Flynn, former campaign manager Paul Manafort, former foreign policy adviser Carter Page and longtime ally Roger Stone. All have denied any wrongdoing.

In his testimony before the Senate Judiciary Committee last week, Comey confirmed that FBI was continuing to pursue questions about possible collaboration between Russia and members of the Trump campaign.

“It was the purpose of the investigation to understand whether there were any coordination or collusion between elements of the campaign and the Russians,” he told the panel.

At the FBI, the news of Comey’s abrupt departure was greeted with shock, said Richard Frankel, a former FBI official who is now an ABC News consultant.

According to Frankel, the agents in charge of the Russia investigation are likely wondering whether “somebody’s going to shut us down, somebody’s going to hamper us, somebody’s going to do something to make this case go away.” He said the agents know that the new director, who will be appointed by Trump and must be approved by the Republican-controlled Senate, will be able to take a range of actions that could hamper their work.

Comey’s successor could demand an immediate review of the case. There could be a request for new agents to join the effort. Or the new director could shut it down entirely.

“There are ways to slow down or detour investigations,” Frankel said.

According to John Carlin, a former assistant attorney general for national security and an ABC News contributor, a move to shutter the investigation would make the country vulnerable to attempts to interfere with U.S. elections and undermine public faith in the rule of law.

“To shut down this investigation would send a message to Russia that it’s game on in terms of their attempts to interfere with our democracy, with democracies throughout the world,” he said. “And it would send a terrible message to the American people. We’ve always been so proud of the integrity of our institutions.”

Blumenthal said he believes the only remedy to protect the integrity of the investigation will be to require the attorney general to appoint an independent prosecutor.

“The FBI can do its job, but it needs a leader,” he said. “The way to assure credibility and integrity in this investigation is to make that leader independent of the president, who potentially is a target here, and that may well be the reason that the president fired this director.”

Calls for an independent investigation came from both sides of aisle, with Sen. John McCain, R-Ariz., tweeting, “Removal of Comey only confirms need for select [committee] to investigate #Russia’s interference in 2016 election.”

Trump could veto any legislative effort by Congress to require an outside investigation, but Blumenthal said he hoped public pressure would make that politically impossible.

“My hope is that we will see outcry and outrage from the American people and bipartisan legislation that will persuade the president he has no question and no recourse but to sign that law,” he said.