WASHINGTON — Federal authorities investigating Blackwater Worldwide contractors are returning to Baghdad this week to revisit the scene of a deadly September shooting that left 17 Iraqi civilians dead.

The two-week trip, by eight prosecutors and FBI agents, marks the latest phase in an inquiry that was limited from the start by the government’s promise of immunity for Blackwater bodyguards who provided sworn statements about the shootings.

The gunplay in a crowded Baghdad intersection has strained diplomatic relations and raised questions over whether some contractors can operate without legal consequences.

The Justice Department team left Tuesday night for Iraq, where they planned to interview witnesses and gather evidence from the Sept. 16 shooting. The trip was confirmed by several people close to the case who spoke on condition of anonymity because of the sensitive nature of the investigation.

Since opening a grand jury investigation in November, prosecutors have questioned about 30 U.S. witnesses, including Blackwater security guards and managers, during closed-door grand jury sessions in a Washington courthouse. To accommodate the crush of witnesses, prosecutors took the unusual step of requesting a third day of grand jury time each week.

Unlike U.S. citizens, Iraqi witnesses cannot be forced to testify before a grand jury in the United States. The trip is an opportunity for prosecutors and FBI agents to question witnesses, assess their credibility and consider whether they should testify if criminal charges in the case are ever brought to trial.

Authorities plan to interview about two dozen Iraqis during this trip, many for the first time, according to one person familiar with the investigation. Some Iraqi witnesses spoke to reporters and local police following the shooting, saying Blackwater employees in the convoy were unprovoked when they opened fire in Baghdad’s Nisoor Square.

FBI agents reviewed reports of those interviews but did not question the Iraqi witnesses themselves during the agency’s first trip to Baghdad two weeks after the shooting.

Prosecutors want to know whether Blackwater contractors, who were hired to guard State Department diplomats in Iraq, used excessive force or violated any laws during the shooting. Blackwater has said its guards were ambushed by insurgents in a busy intersection while fleeing a downtown car bombing.

A preliminary State Department report supports that view. But reports issued separately by the Pentagon and the Iraqi government found no evidence of enemy gunfire. The Iraqi report demanded the U.S. turn over the Blackwater guards to face possible trial in Iraq.

Blackwater and other contractors operated in a legal gray area. Officials exempted contractors from prosecution in Iraqi courts in 2004, but it’s also unclear whether they can be charged in U.S. courts.

Prosecutors are investigating whether they can bring charges under the Military Extraterritorial Jurisdiction Act. To do so, prosecutors must show that the State Department contractors were “supporting the mission of the Department of Defense overseas.”

Defense attorneys are expected to argue that guarding diplomats was a purely State Department function, one independent from the Pentagon. Lawyers for Blackwater had no comment about the Iraqi trip.

Blackwater, which is based in Moyock, N.C., is the largest of the State Department’s three private security contractors with nearly 1,000 personnel working in Iraq. The company has earned more than $1 billion from federal contracts since 2001.

© 2008 The Associated Press