Robert Mueller, the special counsel who spent two years digging into Russian interference in the 2016 election, testified to the House Judiciary Committee and the House Intelligence Committee on Wednesday, issued a stark warning about interference in future campaigns.

“Many more countries are developing the capability to replicate what the Russians have done,” Mueller said, adding that the Russians intend to do it again in 2020. “They’re doing it as we sit here.”

Mueller was subpoenaed to testify before both committees, and did so reluctantly, saying that the special counsel’s 448-page report is his testimony. He declined to answer many questions, and often referred questioners back to the report. In his opening remarks, he also said he would not discuss the origins of the FBI investigation into President Trump’s campaign, or Christopher Steele’s dossier, both of which received attention from House Republicans.

Mueller’s testimony left both parties somewhat frustrated. Under questioning from Democrats, he refused to say whether Trump would have been indicted for obstruction of justice were it not for a Justice Department policy against prosecuting presidents while they are in office. In testimony in the morning, Mueller appeared to say that Trump would have been indicted but for the policy. But he pulled back the remark in the afternoon, saying only that his office had reached no conclusion.

Mueller made his sharpest comments about Trump when he was asked to comment on the president’s praise for Wikileaks, which disseminated stolen Clinton campaign emails during the 2016 campaigns.

“Problematic is an understatement in terms of what it displays — in terms of giving some hope or some boost to what is and should be illegal activity,” he said.

At the Judiciary Committee hearing, Chairman Jerrold Nadler opened with a question about the report’s findings on obstruction of justice.

“Did you actually totally exonerate the president?” he asked.

“No,” Mueller responded. “The finding indicates that the president was not exculpated for the acts he allegedly committed.”

The report found there was not enough evidence to prosecute any American for conspiring with the Russian government’s effort to hack emails and interfere with the election. The second volume of the report detailed evidence of Trump’s efforts to stymie the investigation, but did not make a determination about obstruction of justice.

Nadler sought to clarify why the president had not been accused, referring to an Office of Legal Counsel opinion that the president cannot be prosecuted while in office.

“The statement would be you would not indict, and you would not indict because under the OLC opinion a sitting president could not be indicted,” Mueller said.

Mueller later testified that Trump could be charged after he leaves office.

Doug Collins, the ranking Republican on the committee, said he hoped that Mueller’s testimony would bring “closure” to the investigation.

“The president’s attitude toward the investigation was understandably negative, yet the president did not use his authority to end the investigation,” Collins said.

Rep. James Sensenbrenner, a Republican from Wisconsin, noted that the report does not refer to any of the acts as impeachable offenses. But Mueller declined to rule that option out, saying it is beyond his purview.

“We have studiously kept in the center of our investigation our mandate,” Mueller said. “Our mandate does not go to other ways of addressing conduct.”

Under questioning from Intelligence Committee Chairman Adam Schiff, Mueller spoke of the Trump campaign’s openness to assistance from Russia, and the lies the campaign told to cover it up.

“Your investigation is not a witch hunt, is it?” Schiff asked.

“It is not a witch hunt,” Mueller replied.

Schiff asked, “When the president said the Russian interference was a hoax, that was false wasn’t it?”

“True,” Mueller said.