Three security guards working for the private US contractor Blackwater have been found guilty of the manslaughter of a group of unarmed civilians at a crowded Baghdad traffic junction in one of the darkest incidents of the Iraq war.

A fourth, Nicholas Slatten, was found guilty of one charge of first-degree murder. All face the likelihood of lengthy prison sentences after unanimous verdicts on separate weapons charges related to the incident. Lawyers for the guards say they plan to appeal.

The Nisour Square massacre in 2007 left 17 people dead and 20 seriously injured after the guards working for the US State Department fired heavy machine guns and grenade launchers from their armoured convoy in the mistaken belief they were under attack by insurgents.

But attempts to prosecute the guards have previously foundered because of a series of legal mistakes by US officials, and the case had attracted widespread attention in Iraq as a symbol of apparent American immunity.

Now, after a 10-week trial and 28 days of deliberation, a jury in Washington has found three of the men – Paul Slough, Evan Liberty and Dustin Heard – guilty of a total of 13 charges of voluntary manslaughter and a total of 17 charges of attempted manslaughter.

The fourth defendant, Slatten, who was alleged to have been first to open fire, was found guilty of a separate charge of first-degree murder. Slough, Liberty and Heard were found guilty of using firearms in relation to a crime of violence, a charge which can alone carry up to a 30-year mandatory sentence.

Prosecutors had claimed Slatten, the convoy’s sniper, viewed killing Iraqis as “payback for 9/11” and often “deliberately fired his weapon to draw out return fire and instigate gun battles” or tried to smash windscreens of passing cars as his convoy rolled through Baghdad.

Jeremy Ridgeway, another member of the convoy known as Raven 23, pleaded guilty to voluntary manslaughter in 2008 and agreed to testify against his colleagues in exchange for a more lenient sentence.

Although jurors failed to reach a verdict on three of the manslaughter and attempted manslaughter charges relating to Dustin Heard, he was found guilty of the remaining 17, and the near clean-sweep will be seen as overwhelming endorsement of the government’s case that the massacre was unlawful.

Prosecutors told the jury that Slatten triggered the incident by shooting the occupants of a civilian car during a traffic jam at a busy roundabout in Baghdad. As the car rolled forward, other members of the convoy of three armoured vehicles opened fire indiscriminately with heavy weapons claiming they thought they were under attack from an attempted car bombing.

The case hinged on whether or not the defendants’ belief that their convoy was under attack could be justified by limited evidence apparent to them at the time.

Their lawyers focued on the allegedly erratic behavior of one of the vehicles at the junction and evidence of AK-47 bullet casings found nearby, which some witnesses claimed they heard being fired toward the convoy.

Nevertheless, during the trial’s emotional closing arguments, jurors were told of the “shocking amount of death, injury and destruction” that saw “innocent men, women and children mowed down” as they went about their business in downtown Baghdad.

Federal prosecutor Anthony Asuncion said: “These men took something that did not belong to them: the lives of 14 human beings. They were turned into bloody bullet-riddled corpses at the hands of these men.”

“It must have seemed like the apocalypse was here,” said Asuncion in his closing argument, as he described how many were shot in the back, at long range, or blown up by powerful grenades used by the US contractors.

“There was not a single dead insurgent on the scene,” claimed the prosecutor. “None of these people were armed.”

After he described at length the harrowing fate of individual Iraqi civilians attacked by the Blackwater convoy, Asuncion’s voice was shaking, and he was asked to repeat a key line for the court stenographer to hear. “[The witness] opened the door and his son’s brains fell out at his feet,” Asuncion shouted the second time. “As [the witness] put it, ‘the world went dark for me’.”

Dozens of witnesses and relatives from Iraq were flown over for the trial, some showing jurors the scars on their bodies and giving evidence that caused one juror to be recused after she said she could no longer sleep at night.

Prosecutors also recapped evidence from Blackwater colleagues who testified against the accused, claiming they acted with contempt for Iraqi civilians and boasted of turning “a guy’s head into a canoe” and “popping his grape”.

Earlier in the trial, these witnesses had spoken of telling the accused to “cease-fucking-fire” after the attack, which one described as “the most horrible botched thing I have ever seen in my life”.

But defence attorneys argue the men were acting in legitimate self-defence after suspecting a car that was rolling toward them at a busy traffic intersection could contain a bomb.

Blackwater – renamed first Xe Services and then Academi after the incident saw it thrown out of Iraq and dubbed a mercenary force by a United Nations report – reached a civilian settlement on behalf of six of the victims in 2012 and paid an undisclosed sum in compensation.

FBI investigators who visited the scene in the following days described it as the “My Lai massacre of Iraq” – a reference to the infamous slaughter of civilian villagers by US troops during the Vietnam war – in which only one soldier convicted.

Nevertheless, the first attempt to bring the case to trial was thrown out by a judge after it emerged that State Department investigators had promised the defendants that statements made after the attack and leaked to the media would not be used against them in court.

In an unusual political intervention, vice-president Joe Biden promised the US would pursue a fresh prosecution during a trip to Iraq and an appeal court later ruled these errors in witness interviews did not sufficiently taint the evidence to prevent a trial.

Nevertheless, prosecutors were still forced to drop manslaughter charges against Slatten because they had mistakenly exceeded the statute of limitations during the wrangling, and had to pursue tougher charges against him instead because there was no time limit for murder.

The 14 victims killed by the Blackwater guards on trial were listed as Ahmed Haithem Ahmed Al Rubia’y, Mahassin Mohssen Kadhum Al-Khazali, Osama Fadhil Abbas, Ali Mohammed Hafedh Abdul Razzaq, Mohamed Abbas Mahmoud, Qasim Mohamed Abbas Mahmoud, Sa’adi Ali Abbas Alkarkh, Mushtaq Karim Abd Al-Razzaq, Ghaniyah Hassan Ali, Ibrahim Abid Ayash, Hamoud Sa’eed Abttan, Uday Ismail Ibrahiem, Mahdi Sahib Nasir and Ali Khalil Abdul Hussein.