The U.S. Attorney’s office will not prosecute any individuals for graduate student Patrick Casey’s death following a fight in late September, a spokesman confirmed Tuesday.

“The U.S. Attorney’s office worked with the Metropolitan Police Department to thoroughly investigate the death of Patrick Casey and determined that there isn’t sufficient evidence to pursue criminal charges in the case,” spokesman Bill Miller said.

Casey, a 33-year-old Afghanistan war veteran who died of severe head trauma after falling to the ground when someone pushed or punched him, was likely drinking that night, police said in October. They also said he was “irritating” other patrons at the McDonald’s restaurant at 1916 M Street where the fight that led to his hospitalization broke out, but not necessarily instigating an altercation.

MPD began investigating Casey’s death as a homicide shortly after initially labeling the case a felony assault. The department’s spokeswoman, Gwendolyn Crump, declined to comment on the status of that investigation and referred all questions to the U.S. Attorney’s office.

Miller said the office, working with MPD, interviewed witnesses and evaluated physical, documentary, photographic and video evidence before reaching a conclusion.

“Two law enforcement sources, who were not authorized to speak about the case, said the department has closed its investigation, classifying Casey’s death a ‘justifiable homicide by citizen,'” according to The Washington Post.

Casey came to D.C. in August a master’s student at the Elliott School of International Affairs. Friends and family described the 6-foot-4-inch Clifton Park, N.Y. native as “a big teddy bear.”

The University held a memorial service for Casey Oct. 6.