WASHINGTON -- The investigation into Hillary Clinton's email is broadening and is now sure to push deep into 2016.

The former secretary of state has turned over tens of thousands of emails to the FBI -- and has now agreed to hand over her private email server.

The question is whether classified information was put at risk because of her unusual use of the server in her home to conduct State Department business.

For months, Clinton has refused to turn over her private server, and instead provided to the State Department only the messages she considered related to her work as secretary of state -- about half of the 60,000 emails on the server.

"The server contains personal communications from my husband and me, and I believe I have met all of my responsibilities and the server will remain private," Clinton said in March.

But with the FBI now reviewing the security of her email system, Clinton changed course -- agreeing to turn over to the Justice Department the entire server and a thumb drive containing copies of the emails.

Clinton used the private system for all her email, but she has repeatedly denied she used it to send or receive classified information, which can only be transmitted across secure government networks.

"I am confident that I never sent or received any information that was classified at the time it was sent and received," she said.

But the inspector general for the intelligence community found in a limited review of 40 emails that at least four "contained classified information when they were generated" and "should never have been transmitted via an unclassified personal system."

On Tuesday, the State Department said the inspector general determined that two of those four emails "should be upgraded to the top secret level," one of the highest levels of classification.

The emails were not marked as "classified," but not all classified information is stamped "classified."

State Department employees are trained on how to recognize and handle that kind of highly sensitive information -- and are warned not to discuss it outside classified areas.