Clinton's secret e-mails include drone details

The two e-mails on Hillary Clinton's private server that an auditor deemed "top secret" include a discussion of a news article detailing a US drone operation.

A separate conversation could point back to highly classified material in an improper manner or merely reflect information collected independently, US officials who have reviewed the correspondence said.

The sourcing of the information could have significant political implications as the 2016 presidential campaign heats up.