While the U.S. drone program has been widely reported on, the article references classified information and a Clinton adviser follows up "in a way that could possibly be inferred as confirmation," the AP said, citing intelligence officials.

Officials offered the AP different accounts of the second email, saying it could improperly acknowledge classified information or merely show that different individuals separately got the same information.

Clinton, now a Democratic presidential candidate, earlier this week said her aides were handing over her private email server and a backup thumb drive to the Justice Department.

That news came hours after Sen. Chuck Grassley (R-Iowa) sent a memo to a handful of lawmakers confirming that the Office of the Inspector General of the Intelligence Community found four "highly classified" emails out of a batch of 40.

Grassley said that two of the emails were "top secret." Clinton has turned over roughly 30,000 emails in all, and her campaign maintained this week that she did not personally send or receive any emails with classified information, though "some emails that weren't secret at the time she sent or received them might be secret now."