The Justice Department on Thursday brought fresh charges against four former Blackwater Worldwide security contractors, resurrecting a case over a deadly 2007 shooting in Baghdad that inflamed anti-American sentiment in Iraq. The four men, hired to guard American diplomats, are accused of opening fire in Nisour Square on Sept. 16, 2007, killing 17 civilians, including women and children. Prosecutors say the contractors’ convoy used machine guns and grenades in an unprovoked attack. Defense lawyers say they were ambushed by Iraqi insurgents. The guards were charged with manslaughter and weapons violations in 2008, but a federal judge dismissed the case in 2009, ruling that the Justice Department withheld evidence and violated the guards’ constitutional rights. An appeals court reinstated the case in 2011, saying that Judge Ricardo Urbina, now retired, had wrongly interpreted the law. Prosecutors again presented evidence to a grand jury. Blackwater Worldwide is under new ownership and is headquartered in Virginia. It is now called Academi.