Defense lawyers are vowing to appeal the convictions of four former Blackwater security guards after a federal judge handed down lengthy prison terms for their roles in a 2007 shooting of unarmed civilians in Iraq.

Attorneys identified several issues Monday as likely forming the basis of an appeal, including vindictive prosecution and whether State Department contractors could be charged under a federal law that covers the overseas crimes of Defense Department civilian employees.

The move comes after U.S. District Judge Royce Lamberth sentenced former guard Nicholas Slatten to life in prison and three others to 30-year terms for their roles in the shootings that killed 14 Iraqi civilians and wounded 17 others in Baghdad's Nisoor Square.

Slatten, who witnesses said was the first to fire shots in the melee, was sentenced to life after being convicted last October of first-degree murder. The three other guards — Paul Slough, Evan Liberty and Dustin Heard — were each sentenced to 30 years and one day in prison for charges that included manslaughter, attempted manslaughter and using firearms while committing a felony.

The incident strained U.S.-Iraq relations and caused an international uproar over the use of private security guards in a war zone.

Lamberth announced the sentences after a daylong hearing at which defense lawyers had argued for leniency and presented character witnesses for their clients. At the same time, prosecutors asked that those sentences — the minimums mandatory under the law — be made even harsher. He rejected both requests.

"Based on the seriousness of the crimes, I find the penalty is not excessive," Lamberth said.

Appearing in court wearing leg shackles and prison garb, the former contractors insisted they were innocent.

"The verdict is wrong, you know that I am innocent, sir," Slatten told the judge.

"I feel utterly betrayed by the same government I served honorably," Slough said.

But Lamberth said he fully agreed with the jury's guilty verdicts and praised the Justice Department and the FBI for investigating the shooting and putting the truth "out there for the world to see."

All four were convicted in October for their involvement in the killings in the crowded traffic circle in downtown Baghdad. The legal fight over the killings has spanned years.

Prosecutors described the shooting as an unprovoked ambush of civilians and said the men haven't shown remorse or taken responsibility. Defense lawyers countered that the men were targeted with gunfire and shot back in self-defense.

Assistant U.S. Attorney Patrick Martin urged the court to consider the gravity of the crime as well as the sheer number of dead and wounded and "count every victim."

"These four men have refused to accept virtually any responsibility for their crimes and the blood they shed that day," Martin said.

Video monitors in the courtroom showed photos of the dead and wounded, as well as images of cars that were riddled with bullets or blown up with grenade launchers fired by the Blackwater guards.

The defense argued for mercy, saying decades-long sentences would be unconstitutionally harsh for men who operated in a stressful, war-torn environment and who have proud military careers and close family ties. They also argued the guards were using weapons that had been issued by the U.S. State Department for their protection.

"The punishment should be within the limits of civilized standards," defense attorney David Schertler said.

But Lamberth said he would not deviate from the mandatory minimum sentences, noting that similar stiff penalties have been applied to police officers who commit crimes while carrying automatic weapons as part of their jobs.

Mohammad Kinani Al-Razzaq spoke in halting English about the death of his 9-year-old son as a picture of the smiling boy, Ali Mohammed Hafedh Abdul Razzaq, was shown on courtroom monitors.

"What's the difference between these criminals and terrorists?" Razzaq said.

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The sentencing was unlikely to bring an end to the legal wrangling, which began even before the guards were first charged in 2008. A judge later dismissed the case before trial, but a federal appeals court revived it and the guards were indicted again in October 2013.

Even before the trial began, defense lawyers had identified multiple issues as likely forming the basis of an appeal, including whether there was proper legal jurisdiction to charge the defendants in the first place.