Brennan said that he first picked up on Russia's active meddling last summer and, in an August 4, 2016, phone call with Alexander Bortnikov, the head of Russia's FSB intelligence agency, warned him against further interference. Bortnikov, Brennan said, denied any active efforts in the election.

Rep. Mike Turner, an Ohio Republican, grilled Brennan on whether evidence he cited amounted to collusion between Trump aides and Russia.

"Seeing these types of contacts during the same period of time raised my concern," Brennan said.

Brennan cautioned lawmakers that although he could not definitively say if those contacts amounted to "collusion," he knew that Russians were actively cultivating US contacts and, very likely, did not present themselves as Russian spies.

Brennan also said Trump might have broken protocol if he revealed highly classified information with the Russian foreign minister and Russian ambassador to the US in a White House meeting earlier this month.

The panel will get two cracks at Brennan -- the first in public at 10 a.m. ET and the second behind closed doors -- almost two months after his first appearance was dramatically canceled amid the chaos sparked by House intelligence Chairman Devin Nunes' clandestine White House trip.

House investigators are particularly interested in finding out more about how Russia conducted its election attacks inside the US and who Russian spies attempted to recruit to their side, said a House intelligence committee source. Intelligence sources have previously told CNN that Trump foreign policy adviser Carter Page was being cultivating as a source for a Russian spy -- whether he knew it or not. Page has flatly and continually denied that charge

But Brennan's isn't the only high-profile hearing Tuesday. The latest news most likely to hold the Capitol captive is word that Trump asked his own intelligence chiefs -- Director of National Intelligence Dan Coats and National Security Agency Director Mike Rogers -- to rebut Comey's then-public statement that the FBI had opened a criminal probe into Russia's meddling in July of last year.

Coats is testifying in the Senate and Rogers is expected to testify in the House on budget issues, but the blockbuster news of Trump's attempt to curtail a federal probe, first reported by The Washington Post, has already come up.

Meanwhile, Brennan is also likely to face questions about a split among intelligence leaders last summer over the purpose of Russia's meddling in the US election -- whether it was designed to support Trump or merely spur chaos and confusion in the election. Brennan told senior lawmakers as early as last summer that the Russian operation was squarely designed to support Trump.