WASHINGTON (CNN) -- State Department investigators promised Blackwater guards immunity from prosecution for last month's deadly shooting of 17 Iraqi civilians in Baghdad, according to officials familiar with the matter.

That could potentially complicate any attempt to bring criminal charges in the case, the officials said.

The Justice Department and FBI refused comment on the investigation, which the State Department announced in early October. Blackwater also declined comment.

"They were told their statements can't be used against them," said one U.S. government official. "But this doesn't necessarily mean charges can never be brought against these guys."

A second official called the limited immunity "surprising and confusing" and questioned the authority of the State Department's diplomatic security investigators to unilaterally make immunity decisions.

"I can understand there would be a lot of very unhappy people," said a third official, an experienced investigator who said decisions are not usually made without consultation with federal prosecutors.

All the officials refused to be identified because they were not authorized to speak on this sensitive issue.

An FBI team has been in Baghdad investigating the September 16 killings by security contractors hired by the State Department to protect American diplomats in Baghdad.

Security contractors have immunity from Iraqi law under a provision put into place in the early days of the U.S.-led occupation in Iraq.

North Carolina-based contractor Blackwater USA says its guards came under fire while protecting a State Department convoy and acted properly in self-defense. Iraqi authorities have called the killings "premeditated murder."

The State Department had no official comment. But one senior State Department official said it is unclear what transpired, "but whatever it was, it was not something that was sanctioned by the senior management of the State Department." E-mail to a friend

CNN's Elise Labott contributed to this report.

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