D.C. detectives investigating the death of a George Washington University graduate student continued searching Thursday for the people who had been in an altercation with the man.

The victim, Patrick Casey, 33, an Army veteran of the war in Afghanistan, was hurt last Friday in a confrontation at a Northwest Washington fast-food restaurant and died Tuesday afternoon in a hospital, his family said.

Beverly Fields, a spokeswoman for the D.C. medical examiner’s office, said the office had received Casey’s body but had not determined the cause and manner of death. The autopsy was being conducted Thursday, she said.

A police spokeswoman said the investigation is being handled by the department’s homicide unit.

Casey, of Clifton Park, N.Y., was out with friends the night of the incident. He and others got into a cab and went to the 1900 block of M Street NW early last Friday to meet other friends, according to Casey’s parents, Gail and Paul Casey, who said the events were described to them later by a witness.

Three people who the friends did not know were harassing others inside a McDonald’s restaurant and “turned their attention” to Casey and his female friend, Gail Casey said.

Outside the restaurant, the woman was pushed.

Casey — a graduate student in international affairs who stood 6-foot-4 and weighed 280 pounds — tried to defuse the situation by standing 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. A D.C. police report says Casey suffered a severe laceration to the back of his head.

Homicide detectives are seeking to identify the people who instigated the incident, which took place in view of security cameras, authorities said.

Casey joined the Army at age 28 and learned Arabic, although 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 George Washington University’s master’s degree in international studies while searching online.

“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.”