John Kelly, Donald Trump’s chief of staff, may have been using a compromised personal cell phone as far back as December, when the the retired general was still serving as the secretary of the Department of Homeland Security, until he recently turned it over to support staff.

Politico reported late Thursday that the White House I.T. team discovered the suspected breach over the summer after he turned it in to them. The top Trump aide told them the phone “hadn’t been working properly for months,” and that he was having a hard time updating its software; it’s unclear what data, if any, was accessed during that time period. An official said Kelly usually relies on his government-issued phone for work matters, and rarely uses his personal cell.

The detection of the breach incited concerns among Trump staffers, especially given that it coincides with a West Wing e-mail scandal that’s raised its own security questions. As a precaution, storage lockers where staffers can stash their personal devices during work hours have reportedly been added to West Wing

Bill Marczak, a senior research fellow with the Citizen Lab at the University of Toronto, told Politico it wasn’t surprising that Kelly didn’t pick up on the hack. “The average user won’t notice anything at all,” he said. “Really the only way to pick up on that is to do forensics on the phone.”