Sgt. Jose Nazario is charged with manslaughter, assault and use of a firearm in connection with the 2004 killings in Fallujah. Nazario, a former Riverside, Calif., police officer, is on trial in U.S. District Court instead of a military court martial because he has left the Marines.

As he handed the case over to them, U.S. District Judge Stephen Larson told the jurors to bear in mind that "warfare is regulated by law."

Assistant U.S. Attorney Jerry Behnke told the jury in his closing argument they must not let their views about the war or about combat cloud their judgment.

"This case is not about Iraq and whether you support or oppose the war," he said. "As horrible as combat is, while the Marines were under horrible circumstances that morning, that was what they had been trained to handle."

Defense attorney Kevin McDermott countered that convicting Nazario could endanger troops by causing them to second-guess themselves while in battle.

"Do not make the job harder for young men," he said.

Prosecutors played a tape Wednesday of the former squad leader allegedly telling another Marine in 2007 he ordered the killing of four Iraqis.

Nazario allegedly responds to a question from Sgt. Jermaine Nelson by saying "I did," the Los Angeles Times reported.

"What we did wasn't illegal," Nazario also allegedly told Nelson. "You can't play Monday morning quarterback."