Ari S. Zymelman (Courtesy of Williams and Connolly)

Ari S. Zymelman, a lawyer who won court rulings in recent years that had important repercussions for defense contractors accused of abuse and torture during the war in Iraq, died Sept. 15 at the Washington Home hospice. He was 50.

He had lymphoma, said a family friend, Jeff Lang.

Mr. Zymelman was a partner at Williams and Connolly, a District-based firm he joined in 1989. His specialty was business litigation, particularly focused on technology and government contracting.

For nearly the past decade, he was lead counsel for Titan Corp., later acquired by L-3 Services, which provided civilian interpreters to assist U.S. military operations in Iraq.

Scores of Iraqi detainees filed lawsuits in 2004 in U.S. courts against Titan and CACI, a contractor that supplied interrogators to the military. They accused the companies’ employees of complicity in torture and other illegal acts against prisoners at detention facilities in Iraq, including Abu Ghraib prison.

As the cases moved forward, Mr. Zymelman advanced a legal theory that Titan employees were under operational control of the military when conducting their daily activities and that the overriding federal interest in prosecuting an overseas war preempted state tort law claims. The cases were brought in federal courts, which have jurisdiction to adjudicate matters of state law in certain circumstances.

“The substance of the test is simple: Who is telling these people what to do?” Mr. Zymelman said at the time.

F. Greg Bowman, a partner at Williams and Connolly who worked closely with Mr. Zymelman on the Titan case, said his colleague believed that the federal government could punish anyone found to have abused Iraqi detainees through criminal prosecution, that the detainees should seek compensation through an established U.S. claims process, and that injecting state tort law into the process runs counter to important federal interests.

In 2007, U.S. District Judge James Robertson, sitting in the District, largely agreed with Mr. Zymelman’s arguments in dismissing the state tort claims against Titan. Robertson ruled that the record of operational control was clear in Titan’s case but less so with CACI.

In 2009, the U.S. Court of Appeals for the District of Columbia Circuit affirmed the ruling in Titan’s favor and reversed Robertson’s ruling against CACI, saying the Iraq record of both companies supported a finding that state law claims are preempted.

The U.S. Supreme Court declined to review the case and permitted the D.C. Circuit ruling to stand.

A second wave of cases led to numerous legal proceedings. At one point, Mr. Zymelman argued before the full U.S. Court of Appeals for the 4th Circuit, based in Virginia, which ruled that it did not have the jurisdiction at that stage to rule on the merits. The case against Titan was dismissed in October 2012 after reaching a settlement.

However, some of the legal issues that Mr. Zymelman argued successfully before the D.C. Circuit in 2009 remain before the 4th Circuit in the continuing case against CACI and are expected to be heard early next year, Bowman said.

Ari Shlomo Zymelman was born March 4, 1963, in Newton, Mass., and he grew up in Rockville, where he graduated in 1980 from the Charles E. Smith Jewish Day School. He was a 1984 economics graduate of Yale University and a 1988 honors graduate of the University of Chicago’s law school.

He was a Chevy Chase resident and a member of five synagogues: Ohr Kodesh in Chevy Chase; Adas Israel, Kesher Israel and Ohev Sholom, all in Washington; and Beth Israel in Owings Mills.

His first marriage, to Kerry Abelson, ended in divorce. Survivors include his wife of two years, Beth Gansky of Chevy Chase and Owings Mills; two children from his first marriage, Talia Zymelman and Yossi Zymelman, both of Chevy Chase; three stepchildren, Benjamin Gansky of Brooklyn, Mollie Gansky of Rockville and Sophie Gansky of Owings Mills; his parents, Manuel and Nancy Zymelman of Rockville; and a sister.