BAGHDAD - Nearly four weeks after the deadly shootings at a central Baghdad square involving the Blackwater USA private security firm, American military officials and Iraqi investigators say the F.B.I. and State Department are refusing to share information with them on their investigation into the killings.

The American military has not been allowed to speak to Blackwater employees who were in Nisour Square that day, nor have military officials been shown the Blackwater vehicle that the company and State Department officials have said was disabled during the events of Sept. 16 just west of the heavily fortified Green Zone, according to a senior American military officer.

Blackwater provides security here for employees of the State Department, which began an investigation into the shootings last month. But in recent days a team of agents from the Federal Bureau of Investigation has taken the lead in the inquiry and has been asking Iraqi investigators for information about the case.

However, Iraqi investigators say the F.B.I. and State Department have not provided information about the investigation to their Iraqi counterparts, despite repeated requests. A senior Iraqi investigator said that American military officers had also interviewed Iraqi witnesses, collected evidence from Nisour Square and talked to Iraqi investigators.

“We haven’t received any information from the Americans about their own two investigations,” the senior Iraqi investigator said. “F.B.I. investigators have asked us to help them and share our information, as they have started a third investigation.”

The senior American military officer said the State Department had also refused to provide details of its investigation. “We have asked questions,” the official said. “They have not responded back on those.” Both the Iraqi investigator and the American military officer spoke on condition of anonymity because neither was authorized to discuss the investigations publicly.

The State Department’s chief spokesman, Sean McCormack, said State Department and Blackwater officials were cooperating with the investigation but were not authorized to speak directly to the Pentagon or Iraqi authorities about it. The F.B.I. is leading the inquiry, he said.

“It is a matter of bureaucratic arrangement that they have the lead,” Mr. McCormack said. “They are going to exercise their prerogatives with respect to the integrity of the investigation.”

Independent of the events of Sept. 16, the State Department is conducting an internal review of private security contractors’ operations and will share the results as different phases are completed, Mr. McCormack said.

A senior Justice Department official, speaking on condition of anonymity because he was not authorized to discuss the investigation in detail, said it was no surprise that the F.B.I. had not provided information to the military or Iraqi investigators at such an early stage of the inquiry.

“We just got on the ground there,” he said, adding that agents on the case arrived in the country in the last week. “I’m sure at the appropriate levels, liaison is being done. But we don’t keep everyone apprised of every move of our investigation.”

Iraqi officials say that 17 people were killed on Sept. 16 when a Blackwater convoy opened fire on Iraqi motorists and bystanders in Nisour Square.

The senior American military officer said that as far as the military had been able to determine, the Blackwater convoy did not take fire and the shootings were unprovoked. “It was an abuse of force,” the military official said. “There was no imminent threat. We believe innocent people were killed. We believe that things were taken to a level very quickly when it was not necessary.”

In written testimony submitted to Congress but not made public, Blackwater has said that the convoy came under attack by automatic weapons fire. The company has also said it is cooperating with investigators.

An official at the United States Embassy in Baghdad said of the Blackwater employees involved in the events in Nisour Square, “No one has left the country; everybody involved in the incident is here and available.”

Reporting was contributed by James Glanz, Alissa J. Rubin and Qais Mizher from Baghdad, and Philip Shenon and John M. Broder from Washington.

© 2007 The New York Times