Specialist Mario Lozano served in Iraq for 14 months between 2004 and 2005. His memory splits the time into two separate tours of duty: before and after the shots he fired on March 4, 2005.

On that evening, Specialist Lozano, of East Harlem, shot at a car that failed to slow down at a temporary checkpoint on Baghdad’s airport road, known as “Route Irish.” The car contained an Italian journalist, Giuliana Sgrena, and two Italian intelligence officers who had just rescued Ms. Sgrena from her kidnappers. One of the officers, Major General Nicola Calipari, died and instantly became a hero in his country.

The shooting intensified opposition in Italy to the Iraq war, and within two weeks Prime Minister Berlusconi began talking about a timeline for troop withdrawal. An Italian investigation into the shooting culminated in charges of “voluntary homicide” against Specialist Lozano.

A trial, in which Specialist Lozano has no intention of participating, may occur without him in Rome. The proceedings were put on hold last week after Specialist Lozano’s Italian-appointed lawyer argued that Italy lacks jurisdiction to try an American soldier for actions in a combat zone abroad.

In America, the effects of the shooting followed a much shorter trajectory. An American investigation found that Specialist Lozano, then 35, “complied with the rules of engagement” and that before firing he took steps to get the car to stop. But the official absolution did not put an end to his fears.

“I really thought those people in that car were terrorists,” Specialist Lozano said in an interview last week in the office of his lawyer, Edward Hayes. “When they told me who they were, all my strength and all my energy drained out of me. … I started getting really nervous. I was like, ‘Wow, they’re going to hang me.’ I didn’t know who this guy was.”

Specialist Lozano’s platoon had already sustained heavy causalities in his first seven months in Iraq. For the rest of his tour of duty, he coped with an additional set of worries brought on by the checkpoint shooting.

“Weird things go through your mind,” he said. “I was scared. Sometimes I thought they would set me up out there. A little firefight would happen, boom. Or prison. I thought they were going to hand me over to the Italians.”

It was not until his return home, in October 2005, that Specialist Lozano finally was convinced that his government would not turn him over to Italy for prosecution. He said he has been informed that he will not be deployed to Afghanistan, should his unit go there, because of the Italian peacekeeping force in that country.

A native New Yorker, Specialist Lozano was born in Bellevue Hospital. Largely of Puerto Rican ancestry, he shares his name with a paternal grandfather, who was half-Sicilian. Until the checkpoint shooting, Specialist Lozano had intended to travel to Italy someday, he said.

Specialist Lozano left high school in the 10th grade to do construction work, focusing on installing large air-conditioning units. In 1996, he followed this line of work down to Hollywood, Fla., to join his wife and two daughters, and found himself making $10 to $12 an hour, a fraction of the wages he made in New York. He enlisted in the Army and was sent to Alaska, a move he blames for the end of his marriage.

Since his return from Iraq, Specialist Lozano has sought employment in the construction industry but has not managed to keep any job for long. The sleeping pills he now needs to rest often cause him to doze through his alarm, he said.

Through New York’s 69th Infantry Regiment, of which he is a member, he got steady work for seven months guarding John F. Kennedy International Airport. But he quit the job in May, frustrated at what he described as lax perimeter patrols and security in general. When he heard reports last month of a terrorist plot targeting the fuel tanks at the airport, he drove down at lunchtime, when he knew he’d find guards on duty.

From the open window of his truck, Specialist Lozano said he called out, “See? I told you so.”

Although Specialist Lozano avoids reading news about his case in Italy, he has not managed to ignore entirely the claims of Ms. Sgrena, who has written a book on the incident and describes the shooting as an intentional ambush. The journalist, who writes for the Communist paper Il Manifesto, has publicly challenged Specialist Lozano to appear in court in Italy to defend himself. Ms. Sgrena was wounded during the shooting.

Specialist Lozano, who broke his public silence on the shooting in an interview with the New York Post in April, says Ms. Sgrena is responsible for the incident. By reporting from Baghdad without being embedded with a military unit, Ms. Sgrena increased her odds of being kidnapped and put others at risk in a dangerous rescue mission, he said.

But it is Calipari, not Ms. Sgrena, whom Specialist Lozano thinks about most.

“He was a pretty squared-away soldier,” Specialist Lozano said of Calipari, who was 51 when he died. “A good guy. He cared about his country. I feel like I shot one of my own. I try to tell myself that it’s not my fault. But it’s in your head: Why did I have to wind up being the one, why couldn’t it have been someone else?”

Mr. Lozano described one of his nightmares, in which he watches Calipari exit a car flanked by “two big Italian soldiers.” Looking at Specialist Lozano, Calipari asks: “Do you need help, soldier?” At this point, Specialist Lozano says he always wakes up, certain that in the dream’s next frame Calipari would shoot him.

There are waking fears, too. Specialist Lozano is not certain that friends of the dead officer will not seek vengeance. Specialist Lozano’s friends contribute to this fear, he said, by joking about the matter. As a precaution, he said he pays close attention to any strangers he sees in his building in East Harlem. The suspicion he feels at times is so strong that “I’ve almost reached for a weapon,” he said.

And when Specialist Lozano rides in his pickup or on his motorcycle, he said he sometimes accelerates to more than 100 miles an hour to evade tailgaters who seem especially dogged.

“I feel like my future’s uncertain,” he said. “This is a big part of my life right now that I got to put behind me, but I can’t. It’s always popping up. It’s on my mind 24 hours a day. I can’t shake it.”