For generations, freshmen cadets at the United States Military Academy have marked the end of a grueling summer of training with a huge nighttime pillow fight that is billed as a harmless way to blow off steam and build class spirit.

But this year the fight on the West Point, N.Y., campus turned bloody as some cadets swung pillowcases packed with hard objects, thought to be helmets, that split lips, broke at least one bone, dislocated shoulders and knocked cadets unconscious. The brawl at the publicly funded academy, where many of the Army’s top leaders are trained, left 30 cadets injured, including 24 with concussions, according to West Point.

In interviews, cadets who asked that their names not be used for fear of repercussions in West Point’s strictly controlled culture, said the fight had left one cadet with a broken leg and dislocated shoulders in others. One cadet was knocked unconscious and taken away in an ambulance and had not returned to school, they said. But a spokesman for the academy, Lt. Col. Christopher Kasker, said all cadets had returned to duty.

Though talk about the brawl on Aug. 20 had circulated on social media, West Point did not confirm it to The New York Times until Thursday.