In a security briefing days before the shooting, the guards had been told American intelligence indicated that several cars loaded with explosives were roaming Baghdad, looking for targets. One of those, they were told, was a white Kia.

Accounts differ as to how fast the Kia was moving when it broke away from the other cars, but even some Blackwater contractors testifying for the Justice Department said the car made them uncomfortable. Mr. Slough says he fired first, toward the engine block, trying to stop the car. Then several contractors fired, tearing the car apart with machine guns and grenade launchers.

The assault on the Kia is important because it influenced much of the following minutes. If jurors find that Raven 23 reasonably believed it was under attack, they may be more likely to give the guards the benefit of the doubt.

“We can’t sit here after listening to 10 weeks of evidence and second-guess these men for decisions they had to make in a second,” Mr. Schertler said. “Even if they might have been wrong.”

But prosecutors say that Mr. Slatten, not Mr. Slough, fired first. They say he killed the driver with a sniper rifle, causing the driver’s foot to slip off the gas and sending the car creeping forward at idle speed. That “lit the match” that ignited the shooting, prosecutors said, and had Mr. Slatten not fired, the car never would have moved.

“They’re asking you to start the movie five minutes in,” T. Patrick Martin, an assistant United States attorney, said Thursday as the government concluded its closing arguments.

Mr. Slatten is charged with murdering the driver, a charge his lawyer said was unwarranted. “If you don’t have a villain in this story, folks, then what happened with the Kia was a tragic accident,” the lawyer, Thomas G. Connolly, said Thursday.