Former FBI Director James Comey said former Attorney General Loretta Lynch directed him to refer to the Hillary Clinton email probe as a "matter," not an investigation.
He said the request "confused and concerned" him and led him to the decision to make his independent announcement last July about the case.
He said another deciding factor was reports about a meeting on a tarmac between Bill Clinton and Lynch.
Comey said he felt obligated to do something "separately" from the Justice Department in order to "protect the credibility of the investigation."
His remarks came during questioning from committee chairman Sen. Richard Burr (R-NC), who asked why Comey made the announcement.
Comey said the tarmac meeting was the event that "capped" it for him. He said Lynch's direction on how to characterize the email probe was another "brick in the load" in his decision-making process.
At the July 2016 announcement, Comey criticized Clinton for her decision to use a private email server, but explained that he would not recommend prosecution.
In a follow-up later in the hearing, Comey made clear that it was a "criminal investigation" that was taking place. Comey pointed out that the Clinton campaign used "euphemisms [like] security review" to describe the FBI's work.
He said the campaign's descriptions of the FBI investigation were "inaccurate" and "gave [him] a queasy feeling."
Comey is testifying today on his private conversations with President Trump and the investigation into possible collusion between Russia and the Trump campaign.
He testified earlier that the Trump administration "defamed" him after his May 9 firing by claiming that the FBI was in "disarray" and lacked strong leadership.
Watch the clip above and watch the hearing LIVE, here.