FBI Director James Comey appeared on Capitol Hill on Wednesday to testify before the Senate Judiciary Committee.

Comey spoke on a range of subjects, from his letter to Congress days before the 2016 election that many pundits believe swung it to Donald Trump, to Russia's active measures to disrupt the election, to background checks on firearms.

Here are some of the biggest takeaways.

Comey said he feels "mildly nauseous" about his letter on Hillary Clinton's emails influencing the election, but says he woudn't change a thing.

Saying he was put in a position between "speaking and concealing," Comey detailed how Clinton aide Huma Abedin had forwarded classified emails to her husband, embattled former Congressman Anthony Weiner. The FBI found the emails in an investigation into Weiner's alleged impropriety with an underage girl, and Comey believed he had a duty to inform Congress that his separate investigation into Clinton's handling of classified material had to be re-opened.

"There were thousands of Secretary Clinton' s emails on that device, including what they thought might be the missing emails from her first three months as Secretary of State," he explained. "If there was evidence she was acting of bad intent, that's where it would be." 

The investigation was dropped due to a lack of malicious intent on the part of Clinton and Abedin. Comey went on to say he felt sickened at the idea of the normally apolitical FBI throwing an election. But he stood by his actions. "I've gotten all kinds of rocks thrown at me and this has been really hard, but I think I’ve done the right thing at each turn," he told the committee.

The FBI is actively investigating possible coordination between the Trump campaign and Russia.

Comey called Russia the "greatest threat of any nation on earth," and confirmed that the FBI is actively investigating Russia's hacking of U.S. political institutions and whether Trump's campaign was involved. But he refused to say much more than that, including whether the FBI had access to Trump's tax returns. 

"We're not gonna say another peep about [the investigation] until we're done," he insisted, warning the committee not to "over-interpret" his lack of answers. He did admit he'd been interviewed by the Inspector General's office, and likely would be again. 

Bill Clinton's tarmac meeting with Loretta Lynch changed his thinking.

The now-notorious June 2016 airport tarmac meeting between Clinton and then-Attorney General Lynch had a major impact on Comey's handling of the Clinton email probe. 

"A number of things had gone on which I can't talk about yet, that made me worry that the leadership could not credibly complete the investigation and decline prosecution without grievous damage to the American people's confidence in the justice system," Comey told the Senate committee.

"And then the capper was —  and I'm not picking on the attorney general, Loretta Lynch, who I like very much — but her meeting with President Clinton on that airplane was the capper for me," Comey testified, "and I then said, you know what, the department cannot, by itself, credibly end this." 

Russian active measures are still at work trying to subvert U.S. politics.

South Carolina Sen. Lindsey Graham asked Comey if it was "fair to say that the Russian government was still involved in American politics." Comey gave a curt "yes" to the question, as he did to a follow-up question on whether the FBI was working to prevent new attacks.

Homegrown violent extremism is a big problem.

The hearing was actually called by Iowa's Chuck Grassley to investigate a 2015 terror attack in Garland, Texas.

On the subject of terrorism, Comey said the FBI has about 1,000 open investigations pertaining to "homegrown" extremists, and about 300 related to alleged foreign-born terrorists.

Rudy Giuliani might be in a lot of trouble.

Comey confirmed that the FBI is investigating whether there were classified leaks to former New York City Mayor and Trump surrogate Rudy Giuliani about the Clinton probe during the campaign.

When asked about Giuliani’s public claims that he had contacts with FBI agents, Comey replied, “I don't know yet, but there will be severe consequences if people were leaking."