WASHINGTON – Former special counsel Robert Mueller has made few public statements since his May 2017 appointment to investigate Russian interference in the 2016 election and potential coordination with the Trump campaign.

Though the results of his work were made public in a 448-page report in April, many on Capitol Hill and around the country were anxious to hear from him directly about the investigation. After some wrangling, debates and delays, on Wednesday Mueller finally appeared to testify before Congress.

Through it all, the stoic Mueller kept his cool and kept his vow to stick with what was in the text of his report. But there were still some fireworks and moments of humor during his more than four-hour appearance.

Here are some of the highlights:

Robert Mueller testifies:Russian election interference was 'serious,' Trump was 'not exculpated'

A 'most serious' challenge to our democracy

Mueller, who was at the head of the FBI when terrorist attacks killed thousands of Americans on September 11, 2001, said he considered the Russian efforts to sway the 2016 election was one of the worst threats to U.S. he had witnessed.

"Over the course of my career, I’ve seen a number of challenges to our democracy," Mueller said in his opening statement. "The Russian government’s effort to interfere in our election is among the most serious. As I said on May 29, this deserves the attention of every American."

'It is not a witch hunt'

During his afternoon testimony before the House Intelligence Committee, Mueller pushed back against President Donald Trump's charge that the Russia investigation was a "witch hunt."

House Intelligence Committee Chairman Adam Schiff asked Mueller, "When Trump called your investigation a witch hunt, was that false?

Mueller replied, "I'd like to think so, yes."

Schiff asked again, "Your investigation was not a witch hunt, correct," to which Mueller responded, "It is not a witch hunt."

Mueller clarifies why he didn't indict Trump

During his morning session before the Judiciary Committee, Rep. Ted Lieu, D-Calif., asked Mueller if the reason he didn't charge Trump with obstruction of justice was because of a 2000 Justice Department Office of Legal Counsel memo that said, "a sitting President is constitutionally immune from indictment and criminal prosecution."

"That is correct," Mueller replied, apparently making an admission he had previously made a point of denying.

When he began his testimony before the Intelligence Committee later that afternoon, Mueller said that Lieu's assertion that he didn't charge Trump because of the OLC opinion was "not the correct way to say it."

Mueller said that as he stated in the report and at a May 29 news conference, "We did not reach a determination as to whether the president committed a crime."

A president can be charged after leaving office

Mueller told Rep. Ken Buck, R-Colo., that a president could be charged with a crime after leaving office.

Buck asked Mueller about the practices of his special counsel's office and their decision not to charge Trump with the crime of obstruction of justice.

"Could you charge the president with a crime after he left office?" Buck asked.

Mueller responded, "yes."

Mueller was not making a recommendation for or against obstruction charges for Trump but was discussing Justice Department policy.

Mueller tells Jordan eight times he 'can't get into' it

Rep. Jim Jordan, R-Ohio, was an outspoken critic of the Russia investigation and on Wednesday he took the opportunity to demand an answer to "how the false accusations started."

Mueller declined to give him one.

Jordan focused most of his questions on Josephy Mifsud, a London-based professor who told Trump campaign adviser with Russian ties George Papadopolous that the Kremlin "had dirt" on Hillary Clinton.

"I can't get into that," Mueller said when asked if Mifsud lied to his investigators.

"Did you interview Mifsud?" Jordan asked.

"I can't get into that," Mueller said.

Jordan asked if Mifsud was connected to any foreign intelligence agencies.

"I can't get into that," Mueller said. "Can't get into that," he repeated.

"Lot of things you can't get into," Jordan said.

Mueller struggles to answer Collins

One exchange where Mueller seemed to struggle with a lawmaker's question occurred when Rep. Doug Collins, R-Ga., tried to get him to agree that collusion and conspiracy were "synonymous."

"You're gonna have to repeat that for me," Mueller told the congressman, who is known for speaking quickly.

When Mueller said the terms were not interchangeable, Collins grew irritated, asking "Are you sitting here today testifying something different than what your report states?"

After a lengthy exchange, Mueller conceded to "leave it with the report" and Collins said he was hoped they could finally "put to bed the collusion and conspiracy" questions.

Mueller says Trump 'not exculpated'

"The president was not exculpated for the acts that he allegedly committed,” Mueller said during his testimony,

Merriam-Webster quickly chimed in, posting the definition of "exculpate" on its Twitter account.

"To be 'exculpated' is to be cleared from alleged fault or guilt," wrote the dictionary's account post, noting that the term had started trending on social media.

Contributing: Kristine Phillps, Kevin Johnson, Ryan W. Miller and Nicholas Wu