Former CIA Director John Brennan testified on Tuesday that intelligence officials had found "contacts and interaction" between Donald Trump's presidential campaign and agents for the Russian government.

"I'm aware of information and intelligence that revealed contacts and interactions between officials and U.S. persons involved in the Trump campaign that I was concerned about because of known Russian efforts on such individuals," Brennan told members of the House Intelligence Committee.

"And it raised questions in my mind whether or not the Russians were able to gain the cooperation of those individuals," he added. "I don't know whether or not such collusion existed. I don't know. But I know there was a sufficient basis of information to require investigation by the [FBI] to determine whether or not U.S. persons were actively conspiring, colluding with Russian officials."