State’s top diplomatic security official warned Cheryl Mills, Clinton’s chief of staff, against using their BlackBerries in “Mahogany Row,” the suite of offices at the department’s Foggy Bottom headquarters where the secretary and her top deputies worked, in a March 2009 memo.

“I cannot stress too strongly … that any unclassified BlackBerry is highly vulnerable in any setting to remotely and covertly monitoring conversations, retrieving emails, and exploring calendars,” wrote Eric Boswell, who led State’s Bureau of Diplomatic Security.

Clinton’s BlackBerry was not issued by the State Department, according to former spokeswoman Jen Psaki. The Democratic presidential frontrunner received messages to her personal email address on the device. Hundreds of emails sent to that address contained information subsequently deemed classified.