Personal cell phones and other mobile devices were banned from the White House’s West Wing by the Secret Service beginning this month after officials learned Chief of Staff John Kelly’s phone had been compromised.

Trump administration officials “believe” that the White House Chief of Staff John Kelly’s personal cell phone was “compromised” while he was serving as Secretary of Homeland Security, several anonymous U.S. government sources told Politico on Oct. 5

Kelly first notified White House tech support in the summer that his phone had not been working properly, and the staff later investigated and said the device had been compromised. A memo was later sent around the West Wing detailing what happened to Kelly’s phone.

Though technicians are still unsure when Kelly’s phone was initially infected, a September White House memo on the hack confirmed that the former military official had since replaced his personal phone.

A spokesperson for the White House maintained that the former U.S. Marine general had used his government-issued phone rather than his personal one for government affairs and has already switched to a new device to replace the one being investigated. Kelly’s travel schedule prior to his appointment as White House Chief of Staff is now under review in hopes of gaining a lead on where the breach might have occurred.

Mr. Trump regularly mocked former Democratic presidential hopeful Hillary Clinton on the campaign trial for infamously using a private email server to conduct official business while secretary of state, seemingly putting national security at risk by discussing classified intelligence on a platform potentially prone to hacking.