SAN DIEGO (AP) — A Navy SEAL called by prosecutors to testify at the murder trial of a decorated comrade dropped a bombshell Thursday when he acknowledged killing the victim in what he described as an act of mercy for the wounded captive Islamic State fighter.

Special Operator 1st Class Corey Scott said he asphyxiated the adolescent prisoner in Iraq two years ago after Special Operations Chief Edward Gallagher unexpectedly stabbed him. 

A visibly angry prosecutor accused Scott of lying, saying he had told investigators a different story over the months and changed it only after being granted immunity by a judge and ordered to testify. 

The testimony was a major boost to Gallagher, who is fighting charges of premeditated murder in the boy's death and attempted murder in the shooting of civilians. 

Before the stabbing, Gallagher and Scott had stabilized the sedated prisoner who was wounded in an airstrike and he was breathing normally through a tube inserted to clear his airway.