WASHINGTON — For much of the last two years, Senate Intelligence Committee Chairman Richard Burr has been the Russia investigator who is seen but rarely heard on Capitol Hill.

In an exclusive interview with The Associated Press, the North Carolina Republican opened up about the investigation that has now consumed 19 months of his life. He says it has been “frustrating as hell” and much more difficult than he originally envisioned. But he says the integrity of the investigation — and its importance to the institution of the Senate — is something he has labored to protect.

“Nothing in this town stays classified or secret forever,” Burr said. “And at some point somebody’s going to go back and do a review. And I’d love not to be the one that chaired the committee when somebody says, ‘well, boy, you missed this.’ So we’ve tried to be pretty thorough in how we’ve done it.”

Burr said there is “no factual evidence today that we’ve received” on collusion or conspiracy between Russia and President Donald Trump’s campaign. But he said he’s still open on the issue and hasn’t personally come to any final conclusions, since the investigation isn’t finished.

The Senate investigation is the last bipartisan congressional probe of Russia’s interference in the 2016 election and connections to Trump’s campaign.