BAGHDAD, Iraq – FBI agents investigating the September shooting incident involving security contractor Blackwater USA appear focused on whether anyone fired first on the American convoy and have been aggressively gathering ballistics evidence, according to witnesses interviewed by the agents.

In Washington, State and Justice Department officials said the investigation would not be derailed by a reported offer of immunity to the guards. But it remained unclear whether they could be prosecuted under U.S. law for the shooting that killed 17 people.

As anger continued to simmer, the Iraqi government introduced legislation Tuesday stripping contractors of the immunity from Iraqi law they were granted in 2004 by the U.S.-led agency set up to run Iraq immediately after the war. When its investigation is complete, the FBI will submit its evidence to Justice Department officials, who will determine whether to prosecute, a U.S. official familiar with the investigation said.

Whether the convoy was fired upon or threatened in some way before guards began shooting in Nisoor Square on Sept. 16 is likely to be key to that decision, said Scott Silliman, a former military lawyer who is now executive director of the Center on Law, Ethics and National Security at Duke University.

“I think what they’re trying to do is build a case showing the use of force by Blackwater was not justified, and they can do that through witness statements to show that Blackwater and the convoy were not fired upon,” Silliman said.

A U.S. source said the FBI team left Baghdad on Sunday after interviewing dozens of witnesses. The FBI declined to comment on the case, as did a spokeswoman for Blackwater. Blackwater previously has said the guards were responding to what they believed to be enemy fire.

The shooting prompted intense debate about the role that private armed security contractors have played in the Iraq war and the ambiguous legal environment surrounding them.

If the Justice Department decides to prosecute, experts say it would face serious legal hurdles. The Military Extraterritorial Jurisdiction Act permits contractors to be prosecuted for actions in foreign lands if they are working in support of a Defense Department mission. But prosecutors would have to convince a judge that the act also applies to contractors working for the civilian-led State Department.

Since the shooting, the U.S. Congress has passed legislation making all security contractors accountable in U.S. courts, and the State Department has issued new restrictions that will subject their operations to more oversight.

The FBI investigation, undertaken at the request of the State Department, is one of four under way into the shooting incident, which also wounded 24 people.

Iraqi police, the Pentagon and a joint committee of the U.S. Embassy and the Iraqi government also have undertaken inquiries. In a preliminary report, the Iraqi government concluded that the Blackwater guards began firing when a vehicle advanced on the convoy, believing it to be a suicide bomber.

Three witnesses who agreed to speak with the Los Angeles Times after their debriefings with the FBI said the investigators emphasized the importance of whether the security team was fired upon first.

Witnesses said the interviews lasted about two hours. They said agents referred them to a large aerial image of Nisoor Square and asked them to explain how they arrived at the scene, what their vantage point was when the shooting occurred, their detailed recollection of events and what the shooters looked like.

“They were focusing mainly on one thing,” said Mohammed Hafidh Abdul-Razzaq, 37, whose 10-year-old son Ali was shot and killed as he sat in the back seat of their car. “They asked me several times in each interview whether (the guards) were shot at or not.”

Baraa Sadoon Ismail, 29, who still has two bullets and 60 bullet fragments in his abdomen from the shooting, agreed that the agents focused on whether the Blackwater guards shot first.

All the witnesses interviewed by the Times said they told investigators they did not see anyone fire on the private security guards.

“They asked me whether they were exposed to fire,” said Hassan Jabbar Salman, a lawyer who said he was about 20 yards from the guards and was shot four times. “I replied to them that they were never exposed to any kind of fire.”

Investigators have taken possession of at least three cars to gather ammunition rounds still stuck in the seats and upholstery, the witnesses said. Ismail said investigators took three bullet fragments that had been removed from his body to keep as evidence.

Collection of the ballistics evidence, legal experts said, was another way to determine whether Blackwater guards were responding to a threat.

“If the only shell casings found anywhere in the square were those of weapons used by Blackwater, that would tend to support the finding that there was no use of force against the convoy before Blackwater opened fire,” Silliman said.

In Washington, State Department spokesman Sean McCormack told reporters that although State Department investigators could offer witnesses limited protection, they “cannot immunize an individual from federal criminal prosecution.”

“We would not have asked the FBI and the Department of Justice to get involved in a case that we did not think that they could potentially prosecute,” he said.

A Justice Department spokesman, Dean Boyd, concurred. “Any suggestion that the Blackwater employees in question have been given immunity from federal criminal prosecution is inaccurate,” Boyd said.

The interviews have produced some of the most detailed witness accounts to date.

Trapped on all sides by stopped cars, Abdul-Razzaq said he was helpless as gunfire peppered his car. When the security guards left the scene, he said, he ran to another car to check on a shooting victim, only to have his nephew, who had been riding in his car, run up and tell him his own son had been killed.

Abdul-Razzaq ran back. He said he had glimpsed Ali earlier through the rearview mirror, slumped against the door in the back seat, and assumed he must have fainted. But when he opened the door, blood and brain tissue poured from his son’s head. He slammed the door shut in disbelief. Then he jumped into the car, feeling his son’s chest to see if his heart was still beating. The race to the hospital was futile.

Salman and Abdul-Razzaq said they told the FBI they saw victims shot as they tried to turn their cars around and drive away, or even jump out and run.

The American investigators had to bear the brunt of Iraqi rage, and the Iraqis said the agents apologized for the shooting.

“My sister gave them a piece of advice,” said Abdul-Razzaq, whose sister was in the car with him and also gave an interview to the FBI agents. “She said that it would better for them to bomb Iraq with an atomic bomb rather than kill one or two people on a daily basis. “Kill us all in a matter of seconds so that we may be free of this torment.”‘

Witnesses said that they found the agents to be professional and considerate and that they seemed genuinely determined to get to the bottom of what happened. Ismail, who is still unable to walk as a result of his wounds, said he appreciated that the agents brought him a wheelchair to get from the waiting room to the interview room.

“To tell you the truth, I felt they were truly sorry,” Abdul-Razzaq said.

Still, he added: “I told them that if the investigation was not fair, this incident will tarnish the reputation of the American public and put a brand of shame on the forehead of Americans.”

Times staff writers Paul Richter and Peter Spiegel in Washington contributed to this report.

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