A federal jury in Washington convicted four Blackwater Worldwide guards Wednesday in the fatal shooting of 14 unarmed Iraqis, seven years after the American security contractors fired machine guns and grenades into a Baghdad traffic circle in one of the most ignominious chapters of the Iraq war.

The guilty verdicts on murder, manslaughter and gun charges marked a sweeping victory for prosecutors, who argued in an 11-week trial that the defendants fired recklessly and out of control in a botched security operation after one of them falsely claimed to believe the driver of an approaching vehicle was a car bomber. Jurors rejected the guards’ claims that they were acting in self-defense and were the target of incoming AK-47 gunfire.

Overall, defendants were charged with the deaths of 14 Iraqis and the wounding of 17 others at Baghdad’s Nisour Square shortly after noon Sept. 16, 2007. None of the victims was an insurgent.

“This verdict is a resounding affirmation of the commitment of the American people to the rule of law, even in times of war,” said Ronald C. Machen Jr., U.S. attorney for the District, whose office prosecuted the case. “I pray that this verdict will bring some sense of comfort to the survivors of that massacre.”

Defense attorneys appeared stunned and said they would appeal. David Schertler, an attorney for defendant Dustin Heard, called the result “incomprehensible.”

“The verdict is wrong,” Schertler said. “We’re devastated. We’re going to fight this every step of the way.”

The jury of eight women and four men deliberated 28 days before convicting Nicholas A. Slatten, 30, of Sparta, Tenn., of murder. Also convicted were Paul A. Slough, 35, of Keller, Tex., of 13 counts of manslaughter and 17 counts of attempted manslaughter; Evan S. Liberty, 32, of Rochester, N.H., of eight counts of manslaughter and 12 counts of attempted manslaughter; and Heard, 33, of Knoxville, Tenn., of six counts of manslaughter and 11 counts of attempted manslaughter.

Jurors also convicted Slough, Liberty and Heard of using military firearms while committing a felony. Prosecutors dropped three counts against Heard after jurors deadlocked on them.

Slatten faces a mandatory sentence of life in prison for murder. The others — who, like Slatten, are military veterans — face a mandatory minimum sentence of 30 years in prison.

U.S. District Judge Royce C. Lamberth ordered the men held pending sentencing. Jurors declined to comment.

The outcome capped a difficult, years-long quest by prosecutors to bring the case to trial and a milestone in the government’s efforts to monitor security contractors’ conduct on the battlefield.

The contractor shootings and the U.S. government’s refusal to allow the men to be tried in Iraq sent relations between the two countries into a crisis, and the Blackwater name became shorthand for unaccountable U.S. power.

1 of 20 Full Screen Autoplay Close Skip Ad × Jury convicts Blackwater guards in 2007 Iraq deaths View Photos Seven years after American security contractors killed 14 unarmed Iraqis by firing machine guns and grenades into a Baghdad traffic circle, a jury in Washington convicted all four Blackwater Worldwide guards charged in the incident, one of the most ignominious chapters of the Iraq war. Caption American security contractors killed 14 unarmed Iraqis by firing machine guns and grenades into a Baghdad traffic circle. April 13, 2015 Supporters of Blackwater guards gather outside the E. Barrett Prettyman U.S. Courthouse in Washington. The four Blackwater guards were sentenced for 2007 shootings that killed or injured 31 unarmed Iraqis in Baghdad. Astrid Riecken/For The Washington Post Buy Photo Wait 1 second to continue.

The security firm’s founder, Erik Prince, eventually left the company, which was renamed Xe Services, then later sold and renamed Academi.

“We at ACADEMI are relieved that the justice system has completed its investigation” into the tragedy and that any wrongdoing “has been addressed by our courts,” company spokeswoman Callie Wang said in a statement.

In Congress, lawmakers denounced the Blackwater shootings as recently as this summer, but legislation that would provide clearer jurisdiction to prosecute criminal wrongdoing overseas by private contractors has languished for years.

The author of the proposed change, Senate Judiciary Committee Chairman Patrick J. Leahy (D-Vt.), applauded the verdict Wednesday, but added, “It should not have taken this long for justice to be served. . . . Ensuring that our government has the ability to hold government contractors accountable should be a bipartisan issue, and I hope senators of both parties will work together to pass this important reform.”

Prosecutors called 71 witnesses to four for the defense. The witnesses included victims, surviving relatives and nine fellow Blackwater guards. The 30 Iraqis who testified represented the largest number of foreign witnesses to travel to the United States for a criminal trial, prosecutors said.

At the time of the incident, the defendants were among 19 Blackwater guards providing security for State Department officials in Iraq. Their convoy, called Raven 23, was clearing a path back to the nearby Green Zone for another Blackwater team evacuating a U.S. official from a nearby car bombing.

Assistant U.S. Attorney Anthony Asuncion told jurors that Slatten “lit the match that ignited the firestorm,” firing his sniper rifle at the driver of a stopped white Kia sedan, killing him.

Slough, the convoy command vehicle’s turret gunner, joined in as other Raven 23 members fired into stopped traffic and then turned more firepower onto a panicked, fleeing crowd, prosecutors said.

The only damage caused to the convoy’s command vehicle came from shrapnel by an American grenade fired at short range, the government said.

To buttress their case, prosecutors contended that Slatten and Liberty held hostile views about Iraqi civilians and that Slough on occasion fired weapons at Iraqi targets without provocation.

The defendants’ attorneys said their clients acted reasonably at a time when the Iraqi capital was the scene of “horrific threats” from car bombs, am­bushes and follow-on attacks, sometimes aided by Iraqi security forces­­ infiltrated by guerrillas.

At trial, under cross-examination, some former Blackwater employees differed over whether Slatten or others fired the first shots, and some agreed that they heard incoming AK-47 fire.

Charges in the shooting were first brought against six Blackwater employees in 2008, one of whom, Jeremy Ridgeway, pleaded guilty to manslaughter and testified for the government in the current trial. Charges against another man, Donald Ball, were dropped.

A federal judge, however, threw out the other indictments in 2009, saying that prosecutors improperly relied on statements that the guards gave the State Department immediately after the shooting, believing that they would not be used in court. An appeals court reversed that ruling in 2011, enabling prosecutors to obtain fresh indictments.

The remaining four defendants claimed the violence was triggered by Ridgeway, who a prosecutor conceded suffered from post-traumatic stress disorder and “lost it” in Iraq, and by the convoy’s team leader, Jimmy Watson.

Watson and two other members of the Blackwater team were granted limited immunity by the government to testify against their former colleagues.

Wednesday’s verdict by a civilian jury against the contractors marked a striking departure from how the military justice system dealt with U.S. service members accused of killing 25 unarmed Iraqi in a 2005 raid on their home in Anbar province.

Eight U.S. Marines were initially charged in the Haditha incident, but one was acquitted and six others had charges ranging from involuntary manslaughter to assault against them dropped. Ultimately, only one, Staff Sgt. Frank Wuterich, was convicted of negligent dereliction of duty in connection with the incident, receiving a reduction in rank and no jail time.

Julie Tate and Christian Davenport contributed to this report.