Former CIA chief Brennan had concerns about Russian contacts with Trump campaign Brennan testified in front of Congress today.

 -- Former CIA Director John Brennan told Congress that U.S. intelligence found contact between Russian officials and people involved with Donald Trump's campaign at a time in 2016 when the Russians were "brazenly" interfering in the presidential election.

"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 that I was concerned about because of known Russian efforts to suborn such individuals," Brennan said Tuesday at an open session of the House Intelligence Committee. "And it raised questions in my mind again whether or not the Russians were able to gain the cooperation of those individuals."

Brennan added, however, that he did not know whether any collusion resulted from those contacts. Trump has dismissed such a possibility, saying there is no evidence of collusion.

Earlier this month, former Director of National Intelligence James Clapper testified that there was "no evidence" of collusion in the intelligence community's assessment report on Russian interference — which Trump used to defend himself in a tweet.

However, Clapper clarified to ABC News that he had no evidence available at the time — in January, when the report was released — to show there was collusion with the Russians.

"That's not necessarily exculpatory, since I did not know the state of the investigation or the content — what had been turned up with it," he said.

In addition, Brennan said Tuesday that because of the sensitive nature of a counterintelligence investigation, not everything that was shared with the FBI for its investigatory purposes was shared with Clapper.

Brennan testified that there was a "sufficient basis of information and intelligence that required further investigation" by the FBI to determine whether U.S. citizens were "actively conspiring, colluding" with Russian officials.

"I was worried by a number of the contacts that the Russians had with U.S. persons," he said.

Brennan said he was concerned because of tactics that Russians are known to use, including trying to get individuals, including Americans, to act on their behalf. Russian intelligence operatives won't identify themselves as Russians or as agents of the Russian government; they try to develop personal relationships with individuals and then try to get those people to do things on their behalf, said Brennan.

"By the time I left office on Jan. 20, 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," he said.

Asked if Russia's contacts were with official Trump campaign staffers, Brennan repeatedly declined during the hearing to identify individuals because of the classified nature of the information.

A source familiar with the matter told ABC News on Tuesday that National Security Agency Director Mike Rogers was asked by Trump to publicly push back against the FBI probe into Russia's interference in the 2016 election and possible collusion by Trump associates.

Brennan said he was unaware of any efforts the president has made to enlist the support of intelligence community personnel to counter the collusion narrative.

Warning to the Russians

"It should be clear to everyone that Russia brazenly interfered in our 2016 presidential election process and that they undertook these activities despite our strong protests and explicit warning that they not do so," Brennan during his opening remarks at today's hearing.

He testified that in an Aug. 4, 2016, meeting he warned Alexander Bortnikov, the head of Russia's Federal Securities Bureau intelligence service, that any continued interference would destroy near-term prospects for improvement of relations between Washington and Moscow and would undermine the chance of their working together on matters of mutual interest.

Brennan said he warned that if Russia had such a campaign of interference underway, which had already been reported in the press, it would be "certain to backfire."

"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," he said.

According to Brennan, Bortnikov said that Russia was not doing anything to influence the presidential election and that Washington has traditionally blamed Moscow for such activities. Russia has repeatedly denied interfering in the election.

Despite his denial, Bortnikov said he would inform Russian President Vladimir Putin of U.S. concerns, Brennan said.

He added that the meeting was primarily about Syria but that he also told Bortnikov that Russia's continued mistreatment of U.S. diplomats in Moscow was "irresponsible, reckless, intolerable and needed to stop."

In January of this year, a declassified U.S. intelligence report was released that found Putin "ordered" a campaign to influence the election in an attempt to "undermine public faith in the U.S. democratic process."

Russia also sought to denigrate former Secretary of State Hillary Clinton and harm her election prospects and possible presidency, U.S. intelligence agencies found at the time.

Trump's Oval Office meeting with the Russians

Brennan said it is not unprecedented to share intelligence with Russia or other partners. But he said if reports are true that Trump shared information with Russia's Foreign Minister Sergey Lavrov and Ambassador Sergey Kislyak at a White House meeting on May 10, it violated two protocols.

First, classified intelligence of this nature is shared not with visiting foreign ministers or ambassadors but through intelligence channels so that it's handled the "right way" and to make sure it is not exposed, Brennan said.

Second, before a person shares any classified intelligence with foreign partners, it is important to go back to the originating agency to make sure that sharing the language and substance is not going to reveal sources and methods, possibly compromising future collection capability, said Brennan.

He continued, "So it appears as though, at least from the press reports, that neither did it go in the proper channels nor did the originating agency have the opportunity to clear language for it. So that is a problem."

During the meeting, Trump reportedly shared with the Russians intelligence information about ISIS that came from Israel.

He has defended his disclosure, arguing he has the right to share such information with Russia.

On Monday, while visiting Israel, Trump told reporters, "I never mentioned the word or the name Israel. Never mentioned it during our conversation."

Concern over leaks

Brennan said he was "very concerned" about subsequent releases of what appears to be classified information purporting to point to the source of the information.

"These continue to be very, very damaging leaks, and I find them appalling, and they need to be tracked down," he said.

He added that the Russians are "watching very carefully" what's going on in Washington and that they will try to exploit it for their purposes.

In a statement, a White House spokesman said, "This morning's hearings back up what we've been saying all along: that despite a year of investigation, there is still no evidence of any Russia-Trump campaign collusion, that the president never jeopardized intelligence sources or sharing and that even Obama's CIA director believes the leaks of classified information are 'appalling' and the culprits must be 'tracked down.'"

Justin Fishel contributed to this story.