US President Donald Trump caused outrage on Tuesday night by pardoning four mercenaries from the Blackwater security company who were jailed for a massacre known as Iraq’s “Bloody Sunday.”

Nicholas Slatten, Paul Slough, Evan Liberty and Dustin Heard were convicted by a federal jury in 2014 after FBI investigators concluded that 14 of the 17 people who died in the September 2007 shootings in Baghdad’s Nisour Square had been killed unlawfully.

The four were accused of illegally unleashing “powerful sniper fire, machine guns and grenade launchers on innocent men, women and children” and lying that their convoy had come under attack.

Witnesses said that the security guards had opened fire without provocation.

The massacre started with the killing of Ahmed Haithem Ahmed, who was driving his mother Mohassin to collect his pathologist father from the hospital where he worked.

He was shot though the head and slumped forward as he died, causing his foot to press down on the accelerator and move the car forward towards the convoy of armoured vehicles. The guards responded with a barrage of gunfire and grenades that killed 16 people and wounded at least 24.

Mr Ahmed’s mother was shot dead as she cradled her dying son in her arms. The car was riddled with 40 bullet holes and caught fire after an explosive was thrown inside.

Lorry driver Fareed Walid Hassan told of chaos as people desperately tried to flee, with bullets “falling like rain.”

He saw a woman dragging her child away from the scene.

“He was around 10 or 11,” he said. “He was dead. She was pulling him by one hand to get him away. She hoped that he was still alive.”

The guards continued to fire on civilians despite urgent orders from their colleagues to cease fire. They only stopped shooting when another Blackwater operative pointed a gun at the man still shooting and repeated the order to stop.

Jeremy Scahill, author of Blackwater: The Rise of the World’s Most Powerful Mercenary Army, argues that the firm was among a number of private forces that enjoyed immunity from the constraints of traditional armies.

Blackwater’s leadership, Mr Scahill alleges, was motivated by right-wing Republican ideology. Company founder Erik Prince is the brother of Mr Trump’s Education Secretary, Betsy DeVos.

A number of other Trump allies benefited from a flurry of pardons on Tuesday night, including former congressmen Duncan Hunter and Chris Collins, who had pleaded guilty to offences uncovered by Robert Mueller’s corruption investigations, and former campaign aide George Papadopoulos.

Campaigners are hoping that Mr Trump will pardon Wikileaks founder Julian Assange, who faces life in a US prison on espionage charges if extradited from Britain.