In a blow to the prosecution in the George Zimmerman trial, an expert witness for Zimmerman’s defense testified Tuesday that forensic evidence backs up the community watchman’s story that Trayvon Martin was on top of him when Zimmerman shot and killed the teen.

Dr. Vincent Di Maio, a forensic pathologist who has written four books on gunshot wounds, testified that his review of the autopsy on Martin indicated that the teenager was on top of Zimmerman when Zimmerman fired his gun and killed him. That would seem to support Zimmerman’s claims that he was acting in self-defense.

State prosecutors in Florida Circuit Court in Sanford tried to discredit Di Maio, who acknowledged that he was being paid $400 an hour for his expertise by Zimmerman’s legal team.

Di Maio also testified that Zimmerman’s injuries were consistent with sustaining a broken nose. Prosecutors focused in on that injury, getting Di Maio to acknowledge that the blood would have either flowed onto the street or back into Zimmerman’s body depending on how he was lying.

State attorney Bernie de la Rionda suggested that Zimmerman’s defense team was trying to have it both ways regarding the broken nose, pointing out that no blood was found on Martin’s hand.

If the blood went back up Zimmerman’s nose, Di Maio testified that he would have trouble swallowing and talking. De la Rionda suggested that in that case, Zimmerman could not have been screaming for help on a disputed 911 call. The defense claims the screams came from Zimmerman, which would support the self-defense claim. Prosecutors say Martin was the one screaming because his life was in danger.

De la Rionda pointed out that witness Jonathan Good testified that he didn’t hear punches being thrown or a body part hitting a sidewalk; Zimmerman claimed that he sustained those types of injuries.

The prosecutor then asked Di Maio if he relied on Good’s or any other witness’ statements when he examined the autopsy.

Di Maio said he only used Zimmerman’s statement to see if the evidence supported or invalidated his story because witnesses are unreliable.

“I disregarded the witnesses because … witnesses, they’re all over the place,” Di Maio said. “You can’t use witnesses to make autopsy decisions.”

Zimmerman is facing second-degree murder charges in the February 2012 killing of Martin, who was unarmed during the confrontation. The case spurred national debate about gun laws and racial profiling.