Special Counsel Robert Mueller III, who investigated Russia’s alleged interference in the 2016 United States Presidential elections, said on Wednesday that he had resigned from the Justice Department. Mueller added that charging United States President Donald Trump with obstructing the inquiry was not an option for his team, CNN reported.

“If we had had confidence that the president clearly did not commit a crime, we would have said so,” Mueller said. “We did not, however, make a determination as to whether the President did commit a crime.” Mueller said that it is unconstitutional to charge a president with a federal crime while he is in office.

The special counsel added that the US Constitution requires a process other than the criminal justice system to hold officeholders accountable, leaving the door open for Congress to take up the investigation, CNN reported.

House of Representatives Judiciary Chairperson Jerry Nadler, whose committee will lead impeachment proceedings against Trump, said that Congress will now respond to the “crimes, lies and other wrongdoing” of the president. The Democratic Party, which is in the opposition, has a majority in the House of Representatives.

“Robert Mueller’s statement makes it clear: Congress has a legal and moral obligation to begin impeachment proceedings immediately,” Democratic Senator Cory Booker tweeted. The Senate, however, is controlled by the ruling Republican Party.

However, Trump, in a series of tweets, declared that the case was closed. “Nothing changes from the Mueller Report,” he tweeted. “There was insufficient evidence and therefore, in our Country, a person is innocent. The case is closed! Thank you.”

Later, he added: “How do you impeach a Republican President for a crime that was committed by the Democrats? WITCH-HUNT!”.

The White House claimed that Mueller’s statements confirmed that there was no collusion or conspiracy with Russia and no obstruction of justice by the president. “After two years, the special counsel is moving on with his life, and everyone else should do the same,” White House Press Secretary Sarah Sanders added.

The United States Department of Justice had on April 18 released a redacted copy of Mueller’s investigation. According to the Mueller report, Trump said the investigation was “the worst thing that ever happened to me”. “Everyone tells me if you get one of these independent counsels, it ruins your presidency,” Trump said. “It takes years and years and I won’t be able to do anything.”

Attorney General William Barr, who was appointed by Trump, said last month the Mueller investigation found that Trump’s campaign in 2016 did not collude with the Russian government. However, since Mueller did not draw any conclusions on whether Trump illegally obstructed justice, Barr decided to set aside the evidence.