WASHINGTON

As pressure builds on Hillary Clinton to explain her official use of personal email while serving as secretary of state, she faced new complications Tuesday. It was disclosed her top aides are being drawn into a burgeoning federal inquiry and that two emails on her private account have been classified as "Top Secret."

The inspector general for the Intelligence Community notified senior members of Congress that two of four classified emails discovered on the server Clinton maintained at her New York home contained material deemed to be in one of the highest security classifications - more sensitive than previously known.

The notice came as the State Department inspector general's office acknowledged that it is reviewing the use of "personal communications hardware and software" by Clinton's former top aides after requests from Congress.

"We will follow the facts wherever they lead, to include former aides and associates, as appropriate," said Douglas Welty, a spokesman for the State Department's inspector general.

Despite the acknowledgment, the State Department inspector general's office has left numerous unanswered questions, including exactly who and what is being investigated. The office initially declined to comment and referred questions to the Intelligence Community inspector general's office, which said it is not currently involved in any inquiry into aides and is being denied full access to aides' emails by the State Department. Clinton, herself, is not a target.

The expanding inquiry threatens to further erode Clinton's standing as the front-runner for the Democratic presidential nomination. Since her reliance on private email was revealed in March, polls in crucial swing states show that increasing numbers of voters say Clinton is not honest and trustworthy, in part, because of her use of private emails.