WASHINGTON — The United States government’s decade-long attempt to prosecute four Blackwater contractors in the shooting of dozens of unarmed Iraqis in 2007, a case that became a symbol of American power run amok during the Iraq war, was dealt a blow on Wednesday when the retrial of one security guard ended in a hung jury.

The retrial of the former Blackwater contractor, Nicholas A. Slatten, was declared a mistrial by Judge Royce C. Lamberth of the Federal District Court for the District of Columbia after jurors deliberated for 16 days without reaching a verdict.

The deadlocked jury was the latest twist in the government’s saga to hold people to account for the shooting in Baghdad’s Nisour Square, in which at least 31 people were killed or wounded when contractors in armored trucks began firing machine guns and launching grenades into a busy traffic circle.

The firefight was one of the darkest moments of the Iraq war, along with the Abu Ghraib prison abuse scandal and the civilian massacre in Haditha. It caused strife between Washington and Baghdad, with the Bush White House fighting to keep the contractors from being prosecuted in Iraq and asking Iraqis to trust the American court system.