Graduate student Patrick David Casey, an Afghanistan war veteran who boasted a smile his family called contagious, died Tuesday following a physical assault four days prior. He was 33 years old.

A native of Clifton Park, N.Y., Casey intervened in an argument between three individuals outside the McDonald’s restaurant at 1916 M St., NW at about 2:40 a.m. Friday. The verbal fight “escalated into a physical confrontation,” according to police documents.

Witnesses saw a 6-foot suspect – whose gender is not listed in police records – punch Casey, causing him to fall backwards onto the pavement. The police documents also report that the three suspects fled the scene in an unknown direction.

D.C. Fire and EMS officials later transported Casey to GW Hospital to treat gashes to his nose and the back of his head. His mother, Gail Casey, said the hospital tried for four days to keep him alive.

“I hope they catch these people because I don’t want another family to go through this,” she said, adding that her 6-foot-4-inch son, built like a football player, was “a big teddy bear.”

A conclusion on the manner of his death is still pending, Beverly Fields, chief of staff at the D.C. Office of the Chief Medical Examiner, said.

A public information officer for the Metropolitan Police Department declined to comment beyond the police report regarding the search for the three suspects. The McDonald’s restaurant had security cameras that likely caught most of the altercation on tape.

Casey came to the District in August as a master’s student at the Elliott School of International Affairs.

The Rensselaer Polytechnic Institute graduate enlisted in the U.S Army in 2006 and returned from Afghanistan last year.

Gail Casey said her son developed a love for the Middle East after he visited while working for a computer company before joining the army. He spent six months in Israel and also travelled to Egypt, Jordan and Dubai.

Sgt. Jeremy Nevil, 32, said his best friend was intelligent, funny and overall, a good man. After completing basic training together at Fort Benning, Ga., the two went to Afghanistan.

“I mean, people use the term gentle giant, and that was Pat,” Nevil said. “He could take anybody in a fight but my 6-year-old boy turned him to mush.”

“I’m a better man for knowing him, for sure,” Nevil said.

Nevil said he and Casey shared a unique bond as two Army officials who were nearly 10 years older than their peers. They entered the force in their early 30s, while most others were in their early 20s.

“We loved him so dearly and we’re going to miss him a lot,” he said.

Specialist Stephen Atwood, a 23-year-old stationed at Fort Lewis, Wash., said he met Casey in Afghanistan.

Atwood said Casey, a “gargantuan fellow with a giant beard,” inspired him with both his sharp intellect and sense of humor.

“He never [went] anywhere without this infectious grin and this big sort of laugh where he could tell a joke and everyone’s just roaring with laughter,” Atwood said. “Even if he’s just standing still and being quiet, you can tell his mind is racing.”

Atwood said Casey’s Arabic skills inspired him to learn the Afghani language Pashto.

“It just struck me how well he was mastering these languages…I just wanted to be a bit more like him,” Atwood said.

University President Stephen Knapp said he and other University leaders visited Casey’s family to offer support.

“Our thoughts and prayers are with his family and friends during this time of extraordinary loss,” Knapp said. “Patrick’s family has begun to make funeral arrangements, and we will work with them and his friends to memorialize his life.”

Jacqueline Drayer contributed to this report.