Despite their skepticism about the trial, more than two dozen Iraqi witnesses volunteered to travel to Washington to testify. They described a scene of horror and confusion as they took cover from the machine-gun fire coming from American armored trucks. An Iraqi traffic officer described watching a woman cradle her dead’s son’s head on her shoulder, shortly before her own death. A father sobbed uncontrollably as he testified about his 9-year-old son’s death. And witnesses from inside the Blackwater convoy described their former colleagues as firing recklessly on innocent people.

The shooting began shortly after four American armored trucks rolled into Nisour Square on Sept. 16, 2007. The Blackwater contractors said insurgents ambushed them. Their lawyers described the death of innocent civilians as a tragic and unavoidable consequence of urban warfare.

“Nick Slatten is innocent,” his lawyer, Thomas Connolly, said after the verdicts were announced. “We’re disappointed that the jury found otherwise, but the jury’s verdict does not change the reality of what happened — and what didn’t happen — in Nisour Square.”

Prosecutors said the shooting was unprovoked. But with little forensic evidence and no ballistics linking any gunman to any victim, the case came down to the testimony of witnesses. Many told conflicting stories, forcing prosecutors to urge jurors to believe some aspects of their own case and discount others. During their 28 days of deliberations, jurors sent notes to Judge Royce C. Lamberth that hinted they were planning to convict in the case. But the defendants showed little emotion. Three of them arrived late to court. Mr. Heard broke the courtroom silence by popping open a Coke can just before the jury entered. Lawyers, however, said their clients were devastated by the verdicts.

“This was wrong,” said David Schertler, a lawyer for Mr. Heard. “This verdict is incomprehensible.”

The case against the Blackwater guards faced a number of hurdles, many of them of the government’s own making. From the outset, there were indications that State Department officials tried to gather shell casings after the shooting in an effort to protect Blackwater. The State Department also gave the contractors limited immunity after the shooting, which made it significantly harder for the Justice Department to build its case.

A judge threw out all charges in 2009, citing “reckless” government behavior. A new prosecution team salvaged the case but dropped charges against one guard because of a lack of evidence.