Investigators seeking answers about what caused an Alaska Airlines plane’s near-catastrophic midflight blowout on Jan. 5 are working without a key piece of evidence: audio from the flight’s black box cockpit voice recorder.

National Transportation Safety Board Chair Jennifer Homendy said at a press conference the in-flight audio of Alaska Airlines Flight 1282, which was carrying 171 passengers, had been lost amid the chaos of making an emergency landing at Portland International Airport in Oregon 35 minutes into its journey.

“The cockpit voice recorder was completely overwritten. There was nothing on the cockpit voice recorder,” Homendy told reporters.