On the morning of Sept. 9, 1971 — 50 years ago this Thursday — Attica Correctional Facility in upstate New York was a tinderbox ready to explode.

Inmates were angry about lousy conditions and overcrowding. A manifesto demanding improvements was largely ignored. Animosity over a shoving match between a prisoner and a guard, which had occurred a day earlier, escalated tension at the penitentiary near Buffalo.

Finally all hell broke loose over the recreation yard being locked. Agitators knocked down a steel gate — overpowering guards, beating them and stealing their keys. Many of Attica’s 2,200 convicts joined the riot. By 10:30 a.m. prisoners had gained control of D Yard. Some 39 guards and staffers were taken hostage. One officer was severely beaten, almost to death.

Deanne Quinn Miller, author of “The Prison Guard’s Daughter: My Journey Through the Ashes of Attica” (Diversion Books), out Tuesday, remembers the day well. That grievously injured officer, William “Billy” Quinn, was her dad.

“I was pulled from school and taken to my grandparents’ house while my mother went to the hospital,” Miller, 5 years old at the time, told The Post. “Two days later, on September 11, 1971, my father died from brain trauma.”

Now she wants one thing from the state of New York. “We never got an apology,” said Miller. “I’m hoping that, on the 50th anniversary, Gov. Kathy Hochul, who grew up near Attica, will apologize on behalf of the state.”

The Attica uprising raged for five days before officers retook the ­facility with gunfire, resulting in the deaths of 29 prisoners and 10 correction employees; 128 other prisoners were injured. It stands as the bloodiest prison riot in US history.

Quinn — a 28-year-old father of two daughters with a third on the way — was the only employee to die at the hands of prisoners. John Hill and Charles Pernasilice were later found guilty of murder and attempted assault, respectively.

“I felt like I lost a piece of my life,” said Miller, 55. “My father’s death stayed with me, but I never thought there was anything I could do about it.”

In early 2000, though, she heard  that 1,281 former prisoners or their surviving family would share a $12 million settlement from the state  —  for damages incurred during the retaking of Attica.

Among those up for a share were Hill and Pernasilice. “They were in the yard during the retaking,” said Miller. “So legally they were entitled to a share. I understood that. But I didn’t like it.”

She wasn’t alone. A group of 50 former Attica employees and surviving family members of workers killed in the uprising were appalled. In 2000 they formed the group Forgotten Victims of Attica.