WASHINGTON, Aug 16 (Reuters) - U.S. prosecutors have sent letters to six Blackwater Worldwide security guards, possibly as a prelude to indictments for a shooting incident that killed 17 Iraqi civilians in September, The Washington Post reported in its Sunday editions.

The newspaper, citing three sources close to the case, said prosecutors still are considering evidence after a 10-month FBI investigation of the shooting, which broke out as Blackwater guards escorted a U.S. State Department convoy through Baghdad.

The sources said any charges against Blackwater employees probably would come under the Military Extraterritorial Jurisdiction Act, which the Post said has previously been used to prosecute only cases referred to the Justice Department by the Defense Department for crimes committed by military personnel and contractors overseas.

Some legal experts question whether State Department contractors can be prosecuted under the act.

The guards from the North Carolina-based private security firm said they acted lawfully and fired in self-defense but an Iraqi government investigation said there was no provocation.

The Post said the "target letters" sent to the Blackwater employees offer them a chance to contest evidence and present their own version of the incident. Such letters often are a step taken before indictments are issued.

The Post sources, who requested anonymity because of the sensitive nature of the case, said a final decision on indictments might not be made until October.

The September shooting enraged the Iraqi government, which wanted to put contractors under Iraqi legal jurisdiction. Iraqis also were upset in April when the State Department renewed Blackwater's contract to protect U.S. personnel in Baghdad.

The incident also set off debate in Washington on the use of private contractors in war zones.

(Writing by Bill Trott; Editing by Eric Beech)

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