Fox Sports first reported Del Rio's interview with USC.
USC Athletic Director Pat Haden reached out to Broncos executive vice president John Elway for permission to speak with Denver's defensive coordinator at a time that was convenient for the Broncos.
Del Rio interviewed for the Trojans' job Nov. 1.
The next day, Broncos coach John Fox fell ill while golfing in North Carolina and his doctors determined he needed to move up his scheduled heart surgery.
Del Rio was promoted to interim head coach of the Broncos and won his debut last week at San Diego.
AP NFL website: http://www.pro32.ap.org