Capt. Rogelio Maynulet has never said publicly what happened the day his company chased down a sedan south of Baghdad, opened fire, and Karim Hassan Abid Ali Haleji ended up dead.

But a U.S. surveillance drone hovering in the sky caught the incident on videotape. And 1st Lt. Colin Cremin told investigators what he said Maynulet had told him.

The captain had put his weapon to the Iraqi man’s head or neck and pulled the trigger because the man was too wounded to live, Cremin said.

“It was something he didn’t want to do, but it was the compassionate response,” Cremin said in a pre-trial hearing last year.

The drone video and Cremin’s testimony are expected to be key evidence at this week’s court-martial of Maynulet, a once-promising tank company commander and doctor’s son from Chicago now charged in the assault with intent to commit murder and dereliction of duty.

The trial, set to start Monday at Wiesbaden Army Air Field, Germany, has generated interest worldwide, raising questions about the U.S. military’s use of force, how the military handles crimes by its members, what constitutes an illegal killing in war and whether “mercy killings” are morally defensible.

Thousands of people seem to agree that they are, at least in Maynulet’s case. A petition, to be sent to the U.S. Congress and posted on the Internet, to drop all charges against Maynulet bears nearly 3,000 names and messages of support.

“This is an absolute outrage! Where is their reward for defending us so that we may sleep soundly at night?” wrote one petitioner. “Send the skirt at Nellis who was operating the predator to Iraq for a reality check,” wrote another.

But other Americans, including human rights groups, say the idea of “mercy killing” — which has been claimed by at least two other U.S. soldiers convicted in murdering an Iraqi teen — sets a dangerous precedent.

“Mercy killing is a very convenient excuse,” said Marc Garlasco, senior military analyst at Human Rights Watch in New York City.

Maynulet, 30, and by most accounts an officer on the rise, was originally charged with premeditated murder and dereliction of duty. But after last year’s Article 32 hearing, which is similar to a civilian preliminary hearing, the murder charge was reduced to assault with attempt to commit murder. If found guilty, Maynulet could face a wide range of punishment, with no minimum and a maximum of up to 20½ years in prison.

Capt. Will Helixon, Maynulet’s defense attorney, said he was confident his client would be acquitted.

“The law states that a killing that is without legal justification or excuse is a crime,” Helixon said. “Capt. Maynulet maintains his innocence.”

There are legal defenses to intentional homicide, such as self-defense, defense of others, or, in combat, what’s called “justification.”

But “mercy killing” is not a legal defense or excuse in any U.S. code, including the Uniformed Code of Military Justice.

“It just doesn’t exist in the law,” said Neal Puckett, a retired Marine lieutenant colonel and current military defense lawyer based in Virginia.

“War doesn’t make any difference. Military law has not made an exception to the charge of murder for the battlefield.”

It isn’t in the law books, but a “mercy killing” claim can work nonetheless, experts said.

“If he talks about the reasons why he did that, his attorney may convince the jury not to convict him,” Puckett said. “While the prosecutor might be able to prove the charge, the jury can choose — although it’s not authorized — it can choose to not convict.”

That’s called jury nullification.

Even if there is a conviction, if the panel or judge believes the motive was mercy, a soldier’s sentence can be significantly reduced from the usual murder sentence.

Two staff sergeants from the 1st Battalion, 41st Infantry Regiment, were convicted recently of the murder of an Iraqi teenager who was wounded when their unit mistakenly opened fire on a garbage truck. They admitted they had both shot the 16-year-old as he lay wounded, but they said they had killed the youth to end his suffering.

Staff Sgt. Johnny Horne was sentenced to three years. Staff Sgt. Cardenas Alban was sentenced to one year.

“The jury may have thought that if the evidence showed the person was near death and was going to die anyway and maybe it’s preventing suffering, maybe they got credit with that,” Puckett said. “It’s the difference between legally defensible and morally defensible.”

Earl Martin, an associate dean of academic affairs at Texas Wesleyan School of Law, who was an active-duty Air Force officer and now serves in the reserves, said what may seem like “extraordinary” sentences also could reflect the military court’s decisions to recognize the “dynamic, chaotic” war environment and the difficulty of applying criminal statutes meant for civilian life in that confusing, morally ambiguous arena.

Maynulet’s mission that day, May 21, remains classified, but some reports have said the mission was to capture or kill Muqtada al-Sadr, the Shiite cleric whose militia was causing serious American casualties.

According to the military, Maynulet was leading his unit, Company A, 2nd Battalion, 37th Armor Regiment of the 1st Armored Division, on a patrol near Kufa, when soldiers saw the BMW they thought contained Sadr’s driver and an aide. A chase ensued, the U.S. soldiers opened fire, the sedan crashed.

According to one medic’s testimony, by the time he saw Haleji, the sedan’s driver, Haleji already had suffered a mortal head injury. It was then, according to the military, that Maynulet shot Haleji again.

Haleji’s family, according to the Los Angeles Times, disputed that account, claiming that the father of seven, who they said worked as a driver for a Sadr aide, was not mortally wounded by the initial shooting.

Maynulet did not testify last year at his Article 32 hearing. Helixon said no decision had been made whether the captain would testify at his court-martial.

Maj. Gen. Martin Dempsey, commander of the 1st Armored Division, did not say why he reduced Maynulet’s charge after last year’s hearing. But Puckett said the reduction could reflect problems prosecutors might have been facing in convincing the court-martial panel that Maynulet’s alleged shot was the one that killed the Iraqi man.

He said the defense would likely argue that whatever Maynulet did or thought, if Haleji was already dead, it could not be murder.

Part of the evidence is expected to come from the drone videotape. The video was classified at the Article 32 hearing, and the courtroom was cleared of observers when it was shown. But doctors offered differing medical opinions, based on their video viewing, whether Haleji was dead or alive when Maynulet allegedly shot him.

“If all you have is this drone, maybe the picture isn’t good enough,” Puckett said. “Dying is a process.”