WASHINGTON, United States (AFP) – Four former Blackwater security guards were found guilty Wednesday for their roles in a notorious 2007 mass shooting in Baghdad that left at least 14 civilians dead and deepened resentment of America’s involvement in Iraq.

The four ex-employees of the US private security firm were convicted on an array of charges ranging from first degree murder to voluntary manslaughter.

Their convictions followed a two-month trial that heard how they opened fire with sniper rifles, machine guns and grenade launchers in Baghdad’s bustling Nisour Square as they escorted a diplomatic convoy.

The federal court jury in Washington found Nicholas Slatten guilty of first-degree murder. Paul Slough, Evan Liberty and Dustin Heard were found guilty of voluntary manslaughter. A mistrial was declared in relation to certain counts against Heard.

“This verdict is a resounding affirmation of the commitment of the American people to the rule of law, even in times of war,” US Attorney Ronald Machen said in a statement.

“Seven years ago, these Blackwater contractors unleashed powerful sniper fire, machine guns, and grenade launchers on innocent men, women, and children,” he said.

“Today they were held accountable for that outrageous attack and its devastating consequences for so many Iraqi families,” Machen added.

“I pray that this verdict will bring some sense of comfort to the survivors of that massacre.”

The four men were ordered detained in custody ahead of their sentencing, which is yet to be fixed. Lawyers said they would appeal.

“The verdict is wrong, incomprehensible, we’re devastated but we’re gonna fight every step of the way, we still think we’re gonna win, we’re gonna appeal,” David Schertler, defence attorney for Dustin Heard, said.

Slatten, 32, faces a possible life sentence for his first degree murder conviction.

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.

Slatten’s three co-accused face a minimum of 15 years behind bars for each killing.

Slough, 35, was convicted of 13 counts of voluntary manslaughter and 17 charges of attempted murder. Liberty, 32, was found guilty on eight charges of voluntary manslaughter and 12 attempted murders. Heard, 33, was convicted of six voluntary manslaughter charges and 11 attempted murders.

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.

The killings on September 16, 2007 exacerbated Iraqi resentment toward Americans in the country, and was seen by critics as an example of the impunity enjoyed by private security firms on the US payroll in Iraq.

“People who could laugh, who could love, were turned into bloodied, bullet-riddled corpses, people who were not legitimate targets… who were no real threat to them,” federal prosecutor Anthony Asuncion said during the trial.

Blackwater, whose license to work in Iraq was revoked by Baghdad, was renamed Xe Services in 2009 and then Academi in 2011.

Upon President Barack Obama’s arrival in office in 2009, the State Department canceled its contract with the firm.

The State Department said Wednesday the Baghdad shootings had prompted an overhaul of rules governing use of private contractors.

“The Department took a number of steps to strengthen oversight of private security contractors, such as moving quickly to improve investigative policies and strengthening procedures for use of force and less than lethal force by security contractors,” State Department deputy spokesperson Marie Harf said.

Wednesday’s guilty verdicts came after years of legal twists and turns in US courts.

In 2009, a US judge dismissed charges against five former Blackwater employees because certain statements they made immediately after the event could not be used against them.

Two years later, an appeals court reinstated the indictments against four defendants, opening the way for the trial in Washington.

A judge dismissed the case against Slatten in April because of a technicality. Federal prosecutors then refiled the first-degree murder charge against him several weeks