Mueller gives Trump's legal team questions for potential interview: report
John Kelly's phone was breached as early as December: report
Kelly turned his phone in to the White House IT department after he said it wasn't working and the software was not updating correctly, according to the report.
The department concluded the phone had been breached and could not be used anymore.
It is unclear exactly when the phone was compromised, according to Politico.
Other members of the administration became aware of the incident after White House aides reported it in a one-page memo this month.
News of the incident has sparked security concerns that hackers could have retrieved sensitive information from Kelly's phone while he was Homeland Security chief.
Officials told Politico that Kelly had not used the personal phone since joining the administration in January, and has reportedly been using a different phone since. He reportedly relies on his government phone while at the White House.
"Last December, Gen. Kelly's personal phone stopped working and he discontinued its use," a White House spokesperson told The Hill.
The development comes as the White House conducts an internal probe into personal email use among staff.
Reports surfaced last month that Trump's son-in-law Jared Kushner has used a personal email account for White House business.
The New York Times reported last month that at least six members of the Trump administration, including advisers Gary Cohn and Stephen Miller, sent or received government-related emails on person accounts.
Updated: 9:39 p.m.