Washington - A former Blackwater guard was sentenced to life in prison and three others received 30-year sentences on Monday for their roles in a 2007 mass shooting in Iraq that left at least 14 civilians dead.The four ex-employees of the US private security firm were convicted last October on an array of charges ranging from first degree murder to voluntary manslaughter stemming from the incident in Baghdad's Nisour Square.During a two month-trial in US federal court in Washington, a jury heard how the four defendants opened fire with sniper rifles, machine guns and grenade launchers in the bustling square as they escorted a diplomatic convoy.Iraqi officials say 17 civilians were killed in the shooting. A toll compiled by American investigators recorded 14 deaths. A further 18 Iraqis were injured.US federal judge Royce Lamberth sentenced Nicholas Slatten to life in prison on the first-degree murder charge.The other three defendants, Paul Slough, Evan Liberty and Dustin Heard received sentences of 30 years, the minimum by law for the charge of using a machine gun in a violent crime, plus a token additional day for each of the other counts."The wild thing that happened here can never be condoned by the court," Lamberth told the court, saying he had taken into account statements made on behalf of the four defendants.But he added: "It's clear these fine young men just panicked."The killings on 16 September 2007 deepened Iraqi resentment of America's involvement in the country.In final statements, all four defendants protested their innocence and asked for leniency. Slatten asked for the verdict against him to be overturned.But Lamberth said he fully supported the jury's decision.Before the killings, Slatten allegedly told acquaintances he wanted to "kill as many Iraqis as he could as 'payback for 9/11'", according to court documents.