WASHINGTON — A federal appeals court on Friday reopened the criminal case against four former American military contractors accused of manslaughter in connection with a shooting that killed at least 17 Iraqi civilians in Baghdad in 2007.

Criminal charges against the former employees of Blackwater Worldwide had been dismissed in December 2009 by a federal judge in Washington, who criticized the Justice Department for its handling of the case and ruled that prosecutors had relied on tainted evidence.

The three-judge appeals panel disagreed with that decision, and sent the case back on Friday, ordering Judge Ricardo M. Urbina of Federal District Court to review the evidence against each defendant individually.

“We find that the district court’s findings depend on an erroneous view of the law,” the United States Court of Appeals for the District of Columbia ruled. The appeals judges called on the lower court to determine “as to each defendant, what evidence — if any — the government presented against him that was tainted as to him,” and whether that was enough to justify throwing out the charges.