As Staff Sgt. Henry E. Irizarry lay dying last December after a roadside bomb tore apart his Humvee, his platoon fanned out like seasoned homicide detectives to comb a nearby stretch of pasture in the town of Taji north of Baghdad.

Soldiers found a wire leading from the charred blast site to a field, where someone saw a head peeking out above scraggly reeds. The man ignored a warning shot; gunfire directed at him missed. Soldiers gave chase and cornered him in a nearby house, where they took him into detention.

They swabbed his hands with a kit designed to detect explosive residue. He tested positive.

As a result of their efforts -- described in court documents, after-action reports and interviews with several participants -- and the work of a team of Army lawyers assigned to the case, the Central Criminal Court of Iraq on May 25 sentenced Ziyad Hassan to 15 years in prison. It was just the second time that an Iraqi court has found an insurgent guilty of murder in the killing of a U.S. soldier and the first conviction of a roadside bomber.

"It is very, very rare to catch one of these guys and have what we need to nail him," said Lt. Col. John Dunlap, a staff judge advocate with the 256th Brigade Combat Team, which built the case against Hassan.

"They had testimony from coalition witnesses and people who got injured who complained against the one who did the crime," an Iraqi judge who served on the panel that decided the case said on condition of anonymity. Judges have been frequent targets of insurgent attacks.

"There were photographs and sketches in the file and also medical reports. There was lots of evidence, and they are getting better and more experienced at presenting it."

Hassan's conviction was a breakthrough in the military's increasingly successful effort to prosecute those who target its troops, Army lawyers here said. Since March 30, 18 people have received life sentences from the court for crimes related to attacks against coalition forces, according to Maj. J. Ed Christiansen of Task Force 134, which processes detainees and their legal cases. No one previously had been sentenced to life in prison.

The Hassan case has helped bring about more comprehensive training for front-line soldiers in gathering crime-scene evidence and highlights the challenges facing military lawyers in Iraq's legal system, which they are still working to understand. For example, before Hassan was convicted, the Iraqi prosecutor on the case sought to have it dismissed, citing an alibi the defendant had offered.

In recent weeks, lawyers with the Army's 3rd Infantry Division -- to which Irizarry was assigned -- began providing counterinsurgency units with kits containing cameras, explosive detection devices and pens and paper for sketching diagrams of the events. A slide show prepared by judge advocates shows soldiers how to photograph crime scenes and place evidence in plastic bags without smearing fingerprints.

"Hopefully, you are going to see more of these convictions as our lawyers get a better handle on the justice system and our soldiers get more comfortable with how to gather evidence," said Col. William Hudson, staff judge advocate for the 3rd Infantry Division. "People who commit crimes against us and against Iraqi society should be held accountable."

The attack that killed Irizarry, 38, a father of four from the Bronx who had recently resettled in Waterbury, Conn., came on the afternoon of Dec. 3 as four Humvees carrying soldiers from the Manhattan-based 1st Battalion, 69th Infantry Regiment rumbled along a dirt road near Taji, 35 miles north of Baghdad.

"The mood was light-hearted and joking," according to an April 28 affidavit signed by John L. Cushman, a medic who was traveling with Irizarry in the convoy's rear vehicle. "We were all talking of leave plans and making jokes and picking on each other."

The calm was broken by a thunderous crack that lifted the armored vehicle off the ground. A rush of shrapnel and hot smoke came shooting up through the floor and launched Cushman out onto the ground. With his jaw broken and his face bleeding from deep lacerations, Cushman rushed back toward the Humvee. "I looked in and saw SSG Irizarry's right arm mostly missing and his legs dangling by skin from the knees down," he wrote. Sgt. Adrian Melendez, who was also riding in the vehicle, suffered two broken vertebrae in the attack. Spec. Todd Reed, the Humvee driver, had fractures in both legs.

The legal follow-up to the attack took place just outside Baghdad's fortified Green Zone, in a round, bullet-riddled building that once housed President Saddam Hussein's art collection and other valuables. The court hearing the case was created by the U.S.-led occupation authority in 2003 to handle Iraq's most serious criminal cases.

Of the 490 U.S.-held detainees brought before the court, roughly two-thirds have been convicted, according to military data. Most convictions were for relatively minor charges such as illegal weapons possession or illegally crossing into Iraq from other countries.

Procedurally, the court operates differently from those in the United States. Prosecutions unfold in two distinct phases: an investigative hearing in the chambers of one judge, and a trial in one of the building's cavernous courtrooms, where a panel of three judges determines guilt or innocence.

Under the Iraqi system, judges play a dominant role in all aspects of the proceedings, questioning witnesses, rendering a verdict and handing down a sentence. U.S. military lawyers who compile evidence in cases related to U.S. interests are usually permitted to ask questions of the witnesses during a trial's investigative phase, but for the most part they act as advisers to Iraqi prosecutors.

Hassan's trial began on March 15. Five U.S. soldiers testified, and the judge charged him -- along with a brother who was also detained at the scene of the attack -- with terrorism. But a month later, one day before the final phase was to begin, the Iraqi prosecutor told Dunlap, the staff judge advocate, he would seek to have the case dismissed. Three of Hassan's co-workers had come forward to testify that he had been at work when the bombing took place, and the prosecutor said he found their statements credible.

"We definitely lacked an appreciation for some of the subtleties of Iraqi law because we didn't even know when and how they got that testimony in," Dunlap said. "We were jumping out of our skin, but there was nothing we could do. All of a sudden we thought the thing was over."

The investigative judge rejected the prosecutor's move to dismiss the case, but changed the charge from terrorism to the more specific offenses of murder and assault and scheduled a new trial for May 25. With more time to gather evidence, Army lawyers collected sworn statements from Cushman, Melendez and Reed, who were recuperating in hospitals in the United States, and from Irizarry's widow, Jessica.

Lawyers submitted a DVD of photographs set to the song "Wish You Were Here," by the British rock band Pink Floyd. Pictures of a uniformed Irizarry in Iraq are interposed with shots of his ravaged Humvee and his 5-year-old son Jacob standing by a flag-draped casket at his father's funeral.

"We had no idea if this would help or not or if the judge would watch it," Dunlap said. "But we thought it was a powerful message."

When the trial ended, the prosecutor again argued that the case should be dismissed, citing the alibi the defendant had offered. But the three-judge panel sentenced Hassan to 15 years in prison. His brother was acquitted because of a precedent in Iraqi law that absolves people who help family members conceal crimes after they occur, Dunlap said.

"We listened carefully to what the [defendant] said and compared it to all the other witnesses to decide if he was honest," the trial judge said. "We did not believe the guy. We knew he was lying."

Dunlap and other lawyers involved in the case called it an important victory that would embolden their efforts to prosecute insurgents and a demonstration for troops that attackers they detain will not simply end up back on the streets.

But soldiers who served with Irizarry said the conviction gave them little consolation. Posted in the entryway of their Baghdad barracks are photos of 17 soldiers killed since they arrived in Iraq. "You lose a friend and that's it, he's not coming back," said Sgt. Jason Olmo, 30, of Merrick, N.Y., who was riding in a Humvee two vehicles ahead of Irizarry's on the day of the attack. "I'd rather kill the guy who did this myself. He's lucky we didn't get to take him home that day."

Staff Sgt. Henry E. Irizarry was killed when a roadside bomb destroyed his Humvee. A man detained at the scene was tried by an Iraqi court and given a 15-year sentence.