From Halopedia, the Halo wiki
In 2558, Switchback was assigned to investigate a Covenant dig site. Around the time Fireteam Crimson was finishing their mission, Costabile reported the Covenant were using a Harvester and that they were encountering heavy resistance and were in need of reinforcements Fireteam Crimson was closest to their location and was sent in response.. When Crimson arrived, Cara and the rest of Switchback were nowhere to be found, only finding blank IFF tags. While on a mission in the "Warrens," Crimson found Switchback, when questioned on Costabile's status Spartan Gale informed Commander Palmer that she had been killed by Covenant forces.
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