A Pentagon official said 20,000 to 30,000 pages of emails and other documents from Allen's communications with Kelley between 2010 and 2012 are under review. He would not say whether they involved sexual matters or whether they are thought to include unauthorized disclosures of classified information. He said he did not know whether Petraeus is mentioned in the emails.
Allen has denied any wrongdoing.
The new details in the case brought expressions of amazement in Congress, already in an uproar over the Petraeus affair.
"It's just tragic," King said on NBC. "This has the elements in some ways of a Hollywood movie or a trashy novel."
Lawmakers have said they should have been told earlier about the affair and are asking what the FBI knew and when it notified top Obama administration officials.
The White House wasn't informed of the FBI investigation that involved Petraeus until Nov. 6, Election Day, although agents began looking at Petraeus' actions months earlier, sometime during the summer. Senate Intelligence Committee chairwoman Dianne Feinstein said she first learned of the matter from the media late last week and confirmed it in a phone call to the then-CIA director on Friday.