It’s rare for a single quote to tell you something genuinely new and important about a story as big as the Trump-Russia scandal. There are exceptions — and former CIA Director John Brennan just gave us one for the ages.

Testifying in front of the House Intelligence Committee Tuesday, Brennan said that Russia “brazenly interfered” in the 2016 elections and had been in active contact with members of the Trump campaign. Brennan was careful to avoid explicitly saying that the two sides colluded, and said the Trump aides may not have even known the Russians were spies. Then he dropped the hammer.

"Frequently, people who go along a treasonous path do not know they are on a treasonous path until it is too late,” he said.

It’s important to note that Brennan wasn’t literally accusing anyone in the Trump campaign — let alone the president himself — of committing treason, which has so high a bar that almost no one in the history of the US has been tried and convicted of it.

Still, enough evidence of potential wrongdoing has piled up that the Justice Department recently tapped former FBI Director Robert Mueller to lead an expanding probe into the president’s Russia connections. The investigation is already reaching straight into the West Wing: The Washington Post reported late last week that a member of Trump’s inner circle — widely believed to be Jared Kushner, Trump’s son-in-law and main adviser — is a “significant person of interest” for the FBI.

Brennan stopped short of saying that the communications he saw between Russia and the Trump campaign amounted to actual collusion. His overall point was something damning in a different way: Trump aides were in regular contact with Russia and never stopped to ask themselves whom they were actually working with or why Russians would be taking so strong and direct an interest in an American presidential election.

“I know what the Russians try to do. They try to suborn individuals and try to get individuals, including US individuals, to act on their behalf, wittingly or unwittingly,” he told the panel. “I had unresolved questions in my mind as to whether or not the Russians had been successful in getting US persons involved in the campaign or not to work on their behalf.”

That’s the central question facing Mueller as well. The answer could determine the future of the Trump presidency — and whether impeachment, already under discussion, becomes reality.

The former head of the CIA just took a hell of a shot at Donald Trump

John Brennan’s dislike of the president is far from a secret. When Trump likened US spies to Nazis, Brennan said it was “outrageous.” When Trump talked of ripping up the Obama administration’s Iran deal, Brennan said the move would be “the height of folly.” And when Trump stood in front of a wall honoring fallen CIA officers to (falsely) tout the size of his election win, Brennan labeled it a “despicable display of self-aggrandizement.”

But that doesn’t make his comments Tuesday any less striking, or any less significant. Brennan told the panel that he saw signs of Russian attempts to interfere with the election in the summer of 2016. The former CIA chief said he called the head of Russia’s FSB spy agency that August and basically told him to knock it off.

“I said that all Americans, regardless of political affiliation or whom they might support in the election, cherish their ability to elect their own leaders without outside interference or disruption,” Brennan said. “I said American voters would be outraged by any Russian attempt to interfere in election.”

Brennan said FSB Director Alexander Bortnikov denied any wrongdoing — an assertion that he found unconvincing both then and now.

The interference continued, and US intelligence agencies have unanimously concluded that it was part of a concerted, Kremlin-directed effort to help Trump win the White House. The Washington Post has reported that the US intercepted communications in the aftermath of Trump’s surprise win that captured top Russian officials celebrating his victory over Hillary Clinton and congratulating themselves on the outcome.

From the tone of Brennan’s testimony, it was clear that he remains outraged by the Russian meddling and what he sees as the near certainty that Trump aides either knowingly or unknowingly cooperated with the Kremlin. With the Mueller probe gaining steam, the question of whether the Trump campaign was careless or criminal will eventually be answered.