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Democrats lashed out Friday at President Donald Trump after the investigation he has called a "hoax" and a "witch hunt" resulted in the indictment of 13 Russian nationals on charges of interfering in the 2016 election.

"For all of those who have been asking, 'where is the evidence of a crime?' — this is it. This is the criminal conspiracy," Rep. Elijah Cummings of Maryland, the top Democrat on the House Oversight and Government Reform Committee, said in a statement. "Today's indictments show precisely how the Russians worked to help the Trump campaign, in startling and extensive detail."

House Minority Leader Nancy Pelosi, D-Calif., said the investigation must go forward unobstructed and slammed Trump for sidestepping sanctions aimed at punishing Moscow for election meddling, while Senate Minority Leader Sen. Chuck Schumer, D-N.Y., called for the president to implement them "immediately" in a tweet.

Sen. Bernie Sanders, I-Vt., declared it "absolutely imperative" for Mueller's investigation to go forward, but did not comment on the indictment's detail that Russians sought to express support for his candidacy for the Democratic nomination for president on social media as well.

"Do you still think that whole Russia thing is a hoax?" Rep. Mike Capuano, D-Mass., tweeted at the president.

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Trump has repeatedly pushed back on investigations into Russian meddling in the election, raising doubts that it had even occurred. The president's attorney, John Dowd, said Friday that special counsel Robert Mueller "did a great job" and added that he is "very happy for our country."

Trump tweeted in part after the indictments were announced, "The Trump campaign did nothing wrong — no collusion!" The White House echoed the sentiment in a statement.

Rep. Adam Schiff, D-Calif., the top Democrat on the House Intelligence Committee, which has been investigating Russian interference, said the indictments left a key question unanswered.

"While it does not include an allegation that any U.S. persons conspired wittingly with the Russian actors, the indictment leaves open the vital question of whether Americans, including any associated with the Trump campaign, knowingly played a role in Russia’s active measures campaign," he said in a statement.

The indictment mentions that Russians had contact with "unwitting individuals associated with the Trump Campaign," but when asked about the role of Trump staffers, Deputy Attorney General Rod Rosenstein said "there is no allegation in this indictment that any American had any knowledge."

For some, the indictments were a red line.

"At this point, any step President Trump may take to interfere with the special counsel’s investigation — including removing Deputy Attorney General Rosenstein, or threatening to remove special counsel Mueller directly — will have to be seen as a direct attempt to aid the Russian government in attacking American democracy,” the top Democrat on the House Judiciary Committee, Rep. Jerrold Nadler, D-N.Y., said in a statement.

Rep. Joe Kennedy III, D-Mass., tweeted that the president wasn't doing enough to protect future elections.

"If the man elected to defend our democracy and our ballot box is too weak or distracted to do so, Congress, a co-equal branch of government, must fill the void," he wrote in a tweet.

Republicans said the indictments show the U.S. must be vigilant in protecting the election process.

"We have known that Russia meddled in the election, but these indictments detail the extent of the subterfuge," House Speaker Paul Ryan, R-Wis., said in a statement. "Today's announcement underscores why we need to follow the facts and work to protect the integrity of future elections."

Sen. Ben Sasse, R-Neb., said the threat of future Russian interference must be taken seriously and that the indictments put "Moscow on notice" and should be a "wakeup call to Washington," he said.

Hillary Clinton, the president's Democratic opponent in the 2016 election, declined to comment through a spokesman.