For 12 months while he was in the Army in Afghanistan, Patrick Casey’s parents “waited for the call” that he had been killed in combat. It never came.

But on Friday, Gail and Paul Casey learned that their son, who had recently begun an international-affairs master’s program at George Washington University, had been gravely injured in a confrontation on M Street NW that he was trying to mediate. Patrick Casey, 33, was declared dead from head injuries Tuesday.

Patrick Casey was a computer professional, a traveler and a lover of other cultures, his parents said late Wednesday night in a telephone interview. After working in computer systems in Israel, he traveled through Jordan, Egypt and Dubai.

Casey joined the Army at age 28 and learned Arabic, though he later was sent to Afghanistan. He liked the Army and computers but did not think of either pursuit as a lifelong aspiration, his mother said.

He returned to the United States in February and found GW’s program in international studies while searching online.

“He thought . . . he’d have a lot to offer somebody,” Gail Casey said. “He was hoping to make a difference over there.”

After arriving in Washington in late August, “he was happy, he was focused,” his mother said. “He loved GW, he loved Washington, and he made some terrific friends.”

Patrick Casey was out with friends Thursday night. He and others jumped into a cab and went to the 1900 block of M Street NW early Friday to meet other friends, Gail and Paul Casey said a witness told them.

Three people who they didn’t know were harassing people inside a fast-food restaurant and “turned their attention” to Casey and his female friend, Gail Casey said. Outside the restaurant, the woman was pushed. Casey, 6-foot-4 and 280 pounds, tried to defuse the situation by standing in between the two groups.

“At that point, somebody sucker-punches Pat . . . and he went back and fell and hit his side and he went down” on the sidewalk, Gail Casey said.

Casey suffered a severe laceration to the back of his head, according to a D.C. police report.

After four days, Patrick David Casey, of Clifton Park, N.Y., was declared dead at 2:38 p.m. Tuesday, his mother said.

D.C. police are investigating the incident as a felony assault, as police officials had not confirmed his death late Wednesday, said Officer Paul Metcalf, a spokesman.

Police will await an autopsy report to determine how to classify the case, a source said.

The incident was captured on a surveillance video, and investigators were trying to identify any potential witnesses to the altercation.

“It just doesn’t seem fair,” Gail Casey said. “You just don’t survive [war] and die from this. . . . Little did we know that the worst was yet to come.”