A former Blackwater guard has been sentenced to life in prison and three others to 30 years over the killing of 14 Iraqi civilians in 2007.
Nicholas Slatten and three others were convicted last year for the killings in Baghdad's crowded Nisoor Square.
A further 17 Iraqis were injured as the private contractors opened fire to clear the way for a US convoy.
The shootings sparked international outrage and a debate over the role of defence contractors in warfare.
Slatten faced a charge of murder, while the other men faced multiple counts of manslaughter, attempted manslaughter and using firearms while committing a felony.
In a court in Washington DC, Paul Slough, Evan Liberty and Dustin Heard were all sentenced to 30 years in prison for their involvement in the killings.
The men had claimed that they were under fire from insurgents but prosecutors successfully argued what happened was an unprovoked ambush against civilians.
Dozens of victims and witness were flown from Iraq to the US for the trial.
Video monitors in the courtroom showed photos of the dead and wounded, and images of cars shot at and blown up with grenade launchers fired by the guards.
Mohammad Kinani Al-Razzaq's nine-year-old son Ali was killed in the attack.
A picture of the boy smiling was shown on court monitors as Mr Razzaq said: "What's the difference between these criminals and terrorists?''
The sentences were announced following a day-long hearing in which the defendants reaffirmed their innocence and their lawyers unsuccessfully argued for leniency.
Prosecutors argued that the sentences be made even harsher because the ex-guards had never expressed any remorse.
But US District Judge Royce Lamberth rejected both requests.
"Based on the seriousness of the crimes, I find the penalty is not excessive," Judge Lamberth told the court.
Responding to this, Slatten said: "The verdict is wrong, you know that I am innocent, sir."
Meanwhile, Slough's wife, Kirsten, told the BBC her husband made "reasonable judgements based on extensive training and experience".
"He certainly wished things happened differently... But he has not once told me that he thinks he made a wrong decision," she added.