In stunning testimony that may upend the war crimes trial of Navy SEAL Chief Edward Gallagher, a SEAL medic told the court on Thursday that he and not the chief had killed a wounded captive in Iraq.

The medic, Special Operator First Class Corey Scott, testified that he watched Chief Gallagher stab the prisoner, a teenage ISIS fighter, in the neck, but that the stab wound did not appear to be life-threatening. After the chief walked away, Special Operator Scott told the court, he pressed his thumb over the captive’s breathing tube until he died.

“I knew he was going to die anyway, and wanted to save him from waking up to whatever would have happened to him,” Special Operator Scott said, adding that he had seen other captives tortured and killed by Iraqi forces.