In testimony Tuesday morning before the House Intelligence Committee, former CIA Director John Brennan gave the most straightforward explanation to date as to why U.S. intelligence agencies embarked last summer on a now-sprawling investigation into Russian meddling in the 2016 election and potential coordination between Russian officials and individuals connected to the Trump campaign.

In a cold, high-ceilinged, wood-paneled hearing room underneath the U.S. Capitol, Brennan repeated Tuesday what he and many other intelligence leaders have said before, that “Russia brazenly interfered in the 2016 election process.” While he said he does not know whether any Americans engaged in “actively conspiring” with Russia, he cited evidence of contacts between Russian officials and people associated with the Trump campaign troubling enough to warrant a serious investigation.

“I was concerned because of known Russian efforts to suborn such individuals,” he said. “And it raised questions in my mind again whether or not the Russians were able to gain the cooperation of those individuals.”

That cooperation, he noted, could have happened even without the knowledge of the U.S. participants. “Frequently, individuals who go along the treasonous path do not even realize they’re along that path until it gets to be a bit too late,” he said.

Asked if the individuals he was referring to were official members of the Trump campaign or merely people loosely associated with the president, Brennan declined to identify them, citing the classified nature of the intelligence in question.

Though Brennan did not mention any names in particular, recent reports about two individuals in Trump’s inner circle fit the pattern he described. Russian officials have bragged in recent weeks about their attempts to cultivate both Trump national security adviser Mike Flynn and campaign adviser Carter Page and use them to influence Trump.

Brennan reiterated under questioning that interactions between political campaigns and foreign governments are not in and of themselves suspicious, but the fact that these contacts happened at the very time the CIA was learning about Russian attempts to hack and influence the 2016 election raised his suspicions and motivated him to push for a full investigation.

“I don’t have sufficient information to make a determination whether or not such cooperation or complicity or collusion was taking place,” he said. “But I know that there was a basis to have individuals pull those threads.”

Under questioning from Rep. Thomas Rooney (R-FL), Brennan said that he left office with “unresolved questions” about whether Russia had worked with any “U.S. persons” involved in the 2016 campaign to influence the outcome.

Based on what he knows about Russia from decades of work as an intelligence officer, Brennan said: “They try to suborn individuals and try to get individuals, including U.S. individuals, to act on their behalf—wittingly or unwittingly,” he said. “I was worried about the number of contacts that the Russians had with U.S. persons. Therefore, by the time I left office on January 20th, I had unresolved questions in my mind, as to whether or not the Russians had been successful in getting U.S. persons involved in the campaign or not to work on their behalf again either in a witting or unwitting fashion.”