WASHINGTON ― The former head of the CIA said he has seen intelligence about interactions between President Donald Trump’s campaign associates and Russian officials that made him believe there was a need for the ongoing FBI investigation into possible collusion.

“I encountered and am aware of information and intelligence that revealed contacts and interactions between Russian officials and U.S. persons involved in the Trump campaign,” former CIA chief John Brennan told lawmakers on Tuesday during a House Intelligence Committee hearing. By the time he left the CIA on Jan. 20, Brennan continued, he had “unresolved questions” as to whether the Russians were successful in getting Americans “to work on their behalf, again, either in a witting or unwitting fashion.”

Brennan told lawmakers he could not say with certainty whether the president’s campaign associates colluded with Moscow. “But I know that there was a sufficient basis of information and intelligence that required further investigation by the [FBI] to determine whether or not U.S. persons were actively conspiring or colluding with Russian officials,” he testified Tuesday.

Brennan made the disclosure after multiple Republican lawmakers asked Brennan to provide “evidence” that the Trump campaign colluded with Moscow to boost Trump’s chances of winning the 2016 presidential election. In carefully worded responses, Brennan said that, as CIA chief, he dealt with intelligence rather than evidence ― and the intelligence warranted further investigation.

The former CIA chief’s description of unusual interactions between Trump associates and Russian officials matches a series of press reports that have focused on former Trump advisers Carter Page, Roger Stone, Paul Manafort and Michael Flynn. Brennan declined to publicly name the Trump associates who were in contact with Russian officials, citing classification reasons.

Before leaving the CIA, Brennan played a key role in a U.S. intelligence assessment that accused Moscow of meddling in last year’s presidential election with the goal of helping Trump win. There is no reference to collusion with the Trump campaign in the unclassified version of that assessment, which was released in January, days before Trump entered the White House. Brennan was one of several intelligence officials who briefed Trump on the findings, which were endorsed at the time by the CIA, the Office of the Director of National Intelligence, and the FBI. He has also briefed the so-called Gang of Eight, members of Congress who have access to highly sensitive material.

When he first learned of Russian efforts to sway the election last summer, Brennan said, he confronted Alexander Bortnikov, the head of the Russian Federal Security Service, or FSB, and warned him that such an effort would shatter chances for improved U.S.-Russia relations. “I believe I was the first U.S. official to brace Russia on this matter,” Brennan told lawmakers, referring to an August 2016 conversation with Bortnikov. The FSB chief denied the allegations but said he would pass on the warning to Russian President Vladimir Putin, Brennan recounted.

Brennan, who spent 25 years with the CIA, has emerged as a harsh critic of Trump. When Trump compared the intelligence community to Nazi Germany in January before he took office, Brennan, who was still the head of the CIA, called the remarks “repugnant.” On Trump’s second day in the White House, he visited CIA headquarters and boasted about the size of the crowds at his inauguration the previous day. Brennan said Trump “should be ashamed of himself” for his “display of self-aggrandizement” during that speech, according to Nick Shapiro, former CIA deputy chief of staff.

Tuesday’s hearing was part of an ongoing House Intelligence Committee probe into Russian election interference and possible collusion with Trump associates. The Senate and the FBI are running parallel investigations.