Comey explained that the possibility there could have been another encounter was not something he wanted to discuss in the earlier public hearing, according to a source familiar with the briefing.
The information is based in part on Russian-to-Russian intercepts where the meeting was discussed, three sources familiar with the information tell CNN.
But the sources said it is possible the ambassador, Sergey Kislyak, was exaggerating the extent of the encounter.
CNN previously reported exclusively that congressional investigators are examining whether Sessions had an additional private meeting with Russia's ambassador during the presidential campaign, according to Republican and Democratic Hill sources and intelligence officials briefed on the investigation.
The investigators were focusing on whether such a meeting took place April 27, 2016, at the Mayflower Hotel in Washington, DC, where then-candidate Donald Trump was delivering his first major foreign policy address.
In response to that report, the Justice Department said there was no meeting at the Mayflower.
"The facts haven't changed; the then-Senator did not have any private or side conversations with any Russian officials at the Mayflower Hotel," Department of Justice spokeswoman Sarah Isgur Flores said.
The Justice Department reiterated on Thursday no such meeting took place.
An embattled attorney general
Comey's testimony comes amid speculation about Sessions' job security. CNN reported earlier this week
that Trump and Sessions have had a series of heated exchanges in the last several weeks after the attorney general recused himself from the Russia probe, a source close to Sessions said.
A senior administration official said that at one point, Sessions expressed he would be willing to resign if Trump no longer wanted him there.
A Justice Department spokesperson told CNN Friday that Sessions has no plans to resign, repeating a statement from earlier this week that the attorney general is not planning to step down.
In a statement Thursday night pushing back on parts of Comey's testimony, DOJ said the only reason for Sessions' recusal was his connection to the campaign.
"Given Attorney General Sessions' participation in President Trump's campaign, it was for that reason, and that reason alone, the attorney general made the decision on March 2, 2017, to recuse himself from any existing or future investigations of any matters related in any way to the campaigns for president of the United States," the statement said.
The White House had declined at various times over the week to say whether the President still backs his attorney general, though the White House on Thursday finally affirmed
that Trump has confidence in Sessions.