Inmates assumed control of the building at the James T. Vaughn Correction Center on Wednesday, taking four corrections department workers hostage. The inmates told a local newspaper that concerns about their treatment and the leadership of the United States had prompted their actions.
Authorities did not immediately explain how the employee died. They said Delaware State Police entered the building about 5:05 a.m. Thursday and found the man unresponsive. He was pronounced dead about 25 minutes later.
A second Department of Correction employee who had been held hostage was rescued. She is being examined at a hospital.
The prison is in Smyrna, about 15 miles north of the state capital of Dover.
Delaware Gov. John Carney said the priority now is to determine what happened and why.
In a statement released Thursday, the new Democratic governor said officials will "hold accountable anyone who was responsible" for taking the hostages. He said officials will "make whatever changes are necessary to ensure nothing like it ever happens again."
Details of how many inmates were involved and how they took control of the building were not immediately clear. Robert Coupe, secretary of the Delaware Department of Safety and Homeland Security, said authorities don't know "the dynamics of the takeover" or whether inmates had been held against their will.
LISTEN: Exchange between inmate, negotiator in Del. prison hostage situation
A preliminary investigation suggests the disturbance began about 10:30 a.m. Wednesday when a correctional officer inside Building C, which houses more than 100 inmates, radioed for immediate assistance, Delaware State Police spokesman Sgt. Richard Bratz said. Other officers responded to help, and the employees were taken hostage, he said.
A news release from the Delaware Department of Correction said 14 more inmates were released about 12:30 a.m. Thursday from the building where the hostages had been held and were being held elsewhere at the prison. The news release said a total of 46 inmates had been released from the building since the standoff began.
The hostage situation drew dozens of officers and law enforcement vehicles and prompted a statewide lockdown of all prisons. One hostage was released Wednesday afternoon and another was released hours later.
RAW VIDEO: SWAT team arrives at prison in Smyrna, Del.
One of the freed employees was taken to a hospital with injuries that were not life-threatening, authorities said. The condition of the second wasn't immediately available.
Earlier in the day, inmates reached out to The News Journal in Wilmington in two phone calls to explain their actions and make demands. Prisoners funneled the calls to the paper with the help of one inmate's fiancee and another person's mother. The mother told the paper that her son was among the hostages.
In that call, an inmate said their reasons "for doing what we're doing" included "Donald Trump. Everything that he did. All the things that he's doing now. We know that the institution is going to change for the worse."
That caller said education for prisoners was the inmates' priority. They also said they want effective rehabilitation for all prisoners and information about how money is allocated to prisons.
According to the department's website, the prison is Delaware's largest correctional facility for men, with about 2,500 inmates. It houses minimum, medium and maximum security inmates, and also houses Kent County detainees awaiting trial.
It employs 1,500 corrections officers, according to Bruce Rogers, counsel for the Correctional Officers Association of Delaware.
In 2004, an inmate at the Smyrna prison raped a counselor and took her hostage for nearly seven hours, according to an Associated Press report at the time. A department sharpshooter later shot and killed 45-year-old Scott Miller, according to the report, ending the standoff.