According to State Department spokesperson Alec Gerlach, the department sends a letter to Clinton’s legal representative, “[making] an additional request for any records that former Secretary Clinton may have in her possession.” This is due to news reports in September 2015 that additional work-related emails between her and Army General David Petraeus from her first weeks as secretary of state were found. (McClatchy Newspapers, 12/31/2015)
The letter contains a particular request for emails from the first two months Clinton was secretary of state. This is because Clinton claimed that she only started using her private email in March 2009, but in September 2015, some private emails of hers were found from sources other than Clinton dating back to January 2009.
More recently, Clinton’s aides have claimed that Clinton could not turn over emails from this two month period because they had not been captured on her private server. They haven’t explained why this might be. (McClatchy Newspapers, 10/6/2015) It is unknown if any more emails from that time are found by Clinton, and it’s also not known if she still has copies of her emails.