A US appeals court has thrown out the murder conviction of one Blackwater guard and overturned the sentences of three others in connection with the killing of 14 Iraqis in 2007.

Nicholas Slatten, Paul Slough, Dustin Heard and Evan Liberty – former guards for Blackwater, a private military company contracted by the US State Department – were sentenced in 2015 for their involvement in the killings.

Mr Slough, Mr Heard, and Mr Liberty were sentenced to 30 years in prison for multiple counts of manslaughter, attempted manslaughter, and using firearms while committing a felony. Mr Slatten was sentenced to life in prison for murder.

Now, a US appeals court has thrown out Mr Slatten’s murder conviction, saying he should have been tried separately from his colleagues. The three-judge panel ordered a retrial, where Mr Slatten will be allowed to present previously inadmissable evidence.

The court also overturned the three other guards' sentences, saying their length constituted "cruel and unusual punishment”.

The four guards formed part of a Blackwater convoy that travelled through Baghdad’s Nisour Square in 2007. In an effort to clear the square, the guards opened machine gun fire on the civilians gathered there. Witnesses described the scene as "awful" and "like a horror movie". Fourteen Iraqis were killed, and 17 were injured.

The guards argue they were under fire from insurgents. No witness has supported this claim in trial.

The most iconic images from the war in Iraq Show all 20 1 /20 The most iconic images from the war in Iraq The most iconic images from the war in Iraq U.S. Navy Hospital Corpsman HM1 Richard Barnett, assigned to the 1st Marine Division, holds an Iraqi child in central Iraq, March 29, 2003 Reuters The most iconic images from the war in Iraq An explosion rocks Baghdad during air strikes March 21, 2003 Reuters The most iconic images from the war in Iraq An Iraqi woman watches U.N. weapons inspectors leave Saddam airport in Baghdad March 18, 2003 Reuters The most iconic images from the war in Iraq An Iraqi girl holds her sister as she waits for her mother (R) to bring over food bought in Basra March 29, 2003 Reuters The most iconic images from the war in Iraq U.S. Marine Corp Assaultman Kirk Dalrymple watches as a statue of Iraq's President Saddam Hussein falls in central Baghdad's Firdaus Square, April 9, 2003 Reuters The most iconic images from the war in Iraq US Marines kick in a door while securing a building next to the main hospital in central Baghdad April 15, 2003 Reuters The most iconic images from the war in Iraq A soldier of U.S. Army's 4th Infantry Division (Task Force Ironhorse) searches through dense vegetation around the Diala river where Iraqi militants are hiding outside Baquba early November 13, 2003 Reuters The most iconic images from the war in Iraq An Iraqi detainee gestures toward U.S. soldiers through bars of his cell at Abu Ghraib prison outside Baghdad May 17, 2004 Reuters The most iconic images from the war in Iraq Mays, a young Iraqi Shi'ite girl, cries after a mortar shell which landed outside the family's home in a Najaf residential area injured her uncle August 18, 2004 Reuters The most iconic images from the war in Iraq U.S. Marines carry an injured colleague to a helicopter near the city of Falluja, November 10, 2004 Reuters The most iconic images from the war in Iraq An Iraqi man suspected of having explosives in his car is held after being arrested by the U.S army near Baquba, Iraq, October 15, 2005 Reuters The most iconic images from the war in Iraq A wounded Iraqi woman is helped after several bomb attacks in central Baghdad, July 27, 2006 Reuters The most iconic images from the war in Iraq A man runs down a street warning people to flee shortly after a twin car bomb attack at Shorja market in Baghdad, February 12, 2007 Reuters The most iconic images from the war in Iraq An Iraqi girl holds her hands up while U.S. and Iraqi soldiers search her family house in Baquba early June 30, 2007 Reuters The most iconic images from the war in Iraq An Iraqi woman tries to explain that she has nothing to do with illegal fuel as soldiers from the 2nd battalion, 32nd Field Artillery brigade patrol search for illegal fuel sellers in Baghdad August 6, 2007 Reuters The most iconic images from the war in Iraq U.S. President George W. Bush (L) walks in front of Humvees with Defense Secretary Robert Gates (C) and Secretary of State Condoleezza Rice following remarks to the press after nightfall at Al-Asad airbase in Anbar Province September 3, 2007 Reuters The most iconic images from the war in Iraq U.S. soldiers blindfold an Iraqi man after arresting him during a night patrol at the Zafraniya neighborhood, southeast of Baghdad September 4, 2007 Reuters The most iconic images from the war in Iraq An Iraqi baby lies in a cradle while a woman argues with U.S. soldiers of 1/8 Bravo Company searching for weapons, explosives and information about militants in the area during a foot patrol in a neighbourhood of Mosul June 26, 2008 Reuters The most iconic images from the war in Iraq Policemen cry during a funeral of their colleague a day after a bomb attack in Baghdad's Jihad district November 3, 2010 Reuters The most iconic images from the war in Iraq Staff Sgt. Keith Fidler kisses his wife Cynthia, as their son Kolin looks on, during a homecoming ceremony in New York, April 8, 2011 for the New York Army National Guard's 442nd Military Police Company's return from Iraq Reuters

Mr Slatten was convicted for the murder of a man driving a white Kia that pulled into the square. The government maintains that Mr Slatten’s shots ignited the chaos that ensued.

The court, however, pointed to testimony from Mr Slatten’s co-defendant saying he was not the one to fire the first shots. The testimony had been inadmissible at trial.

“The co-defendant’s statements ... strike at the heart of that theory and instead point to the co-defendant, not Slatten, as the Blackwater convoy member who first ‘engaged and hit the driver’,” the court wrote.

The decision renews a years-long court battle over the incident, which raised serious questions about the military's accountability in Iraq.

The four guards were originally charged in 2008, but their convictions were overthrown after a judge ruled that prosecutors had used evidence inadmissible in court. The case was renewed by a federal appeal in 2013, and the men were sentenced in 2014.

Blackwater Worldwide, which had been awarded more than $1bn in government contracts, became the subject of scrutiny during the case. In 2007, Congress passed a bill that would make all military contractors subject to prosecution by US courts. In 2009, the State Department elected not to renew its contract with the private security company.