WASHINGTON — FBI scientists were unable to match bullets from a deadly 2007 Baghdad shooting to guns carried by Blackwater Worldwide security guards, according to laboratory reports that leave open the possibility that insurgents also fired in the crowded intersection.

Five Blackwater guards face manslaughter and weapons charges for their role in the shooting, which left 17 Iraqis dead and inflamed anti-U.S. sentiment abroad. Prosecutors say the contractors launched an unprovoked attack on civilians using machine guns and grenade launchers. The guards maintain their convoy was ambushed by insurgents.

The FBI lab reports, obtained by The Associated Press from someone not involved in the criminal case, allow for both possibilities.

Investigators recovered .30-caliber bullets from a survivor, a Blackwater truck and around Baghdad’s Nisoor Square. Scientists could not determine whether those bullets came from .30-caliber Blackwater machine guns.

The AK-47 rifles favored by many Iraqi insurgents also fire .30-caliber bullets.

Nobody disputes that Blackwater guards fired, but accounts vary on whether the convoy of armored trucks was attacked. Iraqi witnesses and some members of the Blackwater convoy told authorities they saw no insurgent gunfire. Radio logs show Blackwater guards repeatedly reporting incoming fire during a hectic eight minutes in which one truck was disabled.

The government’s case does not hinge on whether Blackwater was fired on, since prosecutors say the guards violated their rules of engagement even if they did take fire. But any evidence that Blackwater was attacked would help the guards argue they fired in self-defense.

The inconclusive lab reports do little to sort out the discrepancies. The documents do not prove the government’s argument that Blackwater was unprovoked. Nor do they prove that Blackwater was attacked.

Instead, the reports reveal the difficulty FBI agents faced in an investigation that began two weeks after the shooting.

In the U.S., investigators would analyze bullets from the dead bodies. But Muslims typically bury their dead within 24 hours, so when the FBI arrived, such analysis was impossible. Iraqi autopsy reports, if they are done, are not usually as thorough as those in the United States. The FBI lab reports give no indication that investigators recovered any bullets from dead bodies.

Neither the Justice Department nor attorneys for the Blackwater guards would discuss the lab reports.

Further complicating things, investigators found Yugoslavian and Chinese cartridge cases in the square. Neither U.S. troops nor Blackwater use such ammunition. But shootings are not uncommon in Nisoor Square and those shells could have been left behind before or after the Blackwater shooting.

“Even if they didn’t find anything except Blackwater cartridge casings, because of how insecure the scene is out there, people could have come in and picked up anything they wanted,” said Michael Haag, a forensic scientist and instructor who reviewed the lab reports for the AP.

FBI scientists also tried to match the bullets to Blackwater ammunition by analyzing and comparing metal compositions. Scientists determined the bullets and bullet fragments recovered from the scene probably came from “several different sources.”

But that, too, is an inconclusive finding, Haag said. Even ammunition pulled from the same box can have different metal compositions. So, the lab’s finding does not prove whether Blackwater was alone in firing, he said.

Gerald Styers, former president of the Association of Firearm and Tool Mark Examiners, said the FBI reports show that while TV scientists solve shootings in an hour, real science is rarely so easy.

“TV tells a different story than real life,” he said. “It’s very difficult to compare badly damaged materials.”

The Blackwater guards are scheduled to go on trial early next year and the results will be closely watched in Baghdad, where Iraqi authorities wanted to prosecute the case.

The Obama administration says it will not renew Blackwater’s security contract next month. The Moyock, N.C.-based company recently changed its name to Xe and announced a management overhaul.