WASHINGTON, Oct. 29 — State Department investigators offered Blackwater security guards immunity during an inquiry into last month’s deadly shooting of 17 Iraqi civilians, government officials said today, calling it a potentially serious investigative misstep that could complicate efforts to prosecute the company’s employees involved in the episode.

The State Department investigators from the agency’s investigative arm, the Bureau of Diplomatic Security, offered the immunity grants even though they did not have the authority to do so, the said the officials, who spoke on the condition of anonymity because the investigation is ongoing. Prosecutors at the Justice Department, who do have such authority, had no advance knowledge of the arrangement, they added.

Most of the guards who took part in the episode were offered what officials described as limited-use immunity, which means that they were promised they would not be prosecuted for anything they said in their interviews with the authorities as long as their statements were true.

The immunity offers were first reported today by the Associated Press.

State and Justice Department spokesmen would not comment on the matter. “If there’s any truth to this story, then the decision was made without consultation with senior officials in Washington,” one State Department official said.