WASHINGTON (CNN) — Federal prosecutors have notified six guards from military contractor Blackwater that they face possible charges in the 2007 shootings of Iraqi civilians in Baghdad, officials familiar with the case said Sunday.

The six guards all received “target letters” from the Justice Department, which has convened a grand jury to hear evidence in the September shootings, the officials told CNN. No final decisions have been made, but target letters often signal that criminal charges are in the works.

The grand jury has heard from about three dozen witnesses, the officials said.

The shootings in Baghdad’s Nusoor Square left 17 people dead and nearly 30 wounded, according to Iraq’s government. Blackwater officials have said its contractors acted in appropriate self-defense when a U.S. diplomatic convoy came under fire. But an Iraqi government commission that investigated the shootings accused the guards of firing on civilians indiscriminately, and the first U.S. soldiers who arrived on the scene told investigators they found no evidence the guards were fired upon, sources familiar with the investigation have previously told CNN.

Blackwater did not respond to calls and e-mails Sunday from CNN. The story was first reported in Sunday’s Washington Post.