Sgt. Steven Floyd, a 16-year veteran of the department, was found unresponsive and was pronounced dead at 5:29 a.m., minutes before authorities secured the scene at the James T. Vaughn Correctional Center in Smyrna, officials said.
Another prison official who had been taken hostage was rescued from the prison's C Building. She was treated at a local hospital.
"She is alert and well," Perry Phelps, the state corrections commissioner, said in a news conference.
Shortly after 10:30 a.m. Wednesday
, a corrections officer radioed for help in a building that houses about 120 inmates. It's unclear what triggered the call for assistance.
It's also unclear what weapons the inmates used. Police were told they had "sharp instruments," according to Robert Coupe, secretary of Delaware's Department of Safety and Homeland Security.
The all-male facility, the largest in the state, was placed on lockdown and surrounded by police. By the time backup arrived, four corrections employees, including a female counselor, had been taken hostage, police said.
Over the course of the standoff, 46 inmates exited the building, officials said, but Coupe warned reporters that all 120 inmates in C Building, including those 46, are still considered suspects.
"I would not say that they were released. They came out of the building," he told reporters.
About 2:30 p.m., one officer was released and taken to a hospital with injuries not considered life-threatening, officials said.
Shortly before 8 p.m., a second officer was let go, Coupe said. A helicopter with CNN affiliate WPVI-TV
showed footage of the second hostage being wheeled out on a stretcher. That person underwent a health evaluation, Coupe said.
At 10:52 p.m., three maintenance workers who had been hiding in the basement made their way to the rooftop, where police were able to rescue them, Phelps said.
During negotiations, the prisoners demanded that the water be turned back on in C Building. Authorities obliged, Coupe said, believing that the demand was related to hydration and hygiene.
But the prisoners used the water to fill metal foot lockers -- roughly 18 by 24 inches in size -- which they used to construct walls barricading the building's entrances, Coupe said.
Negotiations wore on through Wednesday. Prisoners, using a radio taken from a correctional officer, demanded better conditions at the prison, including education, rehabilitation programs and improved training for prison guards, according to radio traffic picked up by CNN reporters.
State and federal negotiators cautioned the prisoners that these were things that would not happen overnight.
The radio traffic also indicated that a negotiator was concerned about a woman being held hostage and repeatedly asked to talk to a male hostage, to no avail.
Coupe said, at one point, authorities became aware that someone was streaming the negotiations on the Internet and put a stop to it.
Prisoners contact paper
Prisoners not only issued demands to negotiators, but they also called The News Journal in Wilmington, Delaware
, according to the newspaper. Several people spoke to a reporter, including a woman identified as a therapist, whose voice was quavering during the call.
A man, who told the paper he was not permitted to provide his name, got on the phone and said he didn't know the identities of the hostage takers because they were wearing masks. The man said he wanted to convey the prisoners' demands, but the connection was choppy and abruptly cut off.
A second man called later and pointed fingers at the newly elected President of the United States.
"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," the newspaper quoted the man as saying. "We got demands that you need to pay attention to, that you need to listen to, and you need to let them know."
He complained of abusive treatment and said the inmates want better education and rehabilitation programs, along with transparency in how tax dollars are spent on those programs.
"If they keep on playing games, (it's) going to get ugly," he told the paper.
During the negotiations, authorities realized the doors to C Building had been barricaded and began efforts to secure a "breach vehicle" to enter the building. None was immediately available, though Maryland State Police said it would provide one, Coupe said.
Before the vehicle could arrive, police decided they could wait no longer. About 5 a.m. Thursday, they used a state Department of Correction backhoe to breach C Building and enter the prison, Coupe said.
A few minutes later, police discovered the female correction official, who indicated "there were inmates that actually shielded this victim," Coupe said. She was not injured but was taken to a hospital for evaluation, he said.
All the prisoners remaining in C Building were removed, searched, sent to an infirmary and put in new cells, he said. Investigators then began "going cell by cell collecting evidence."
The facility, with three narrow hallways and a central hub, is about 90 miles east of Washington, and houses 2,500 inmates who are in minimum-, medium- and maximum-security cells. Death row inmates are also held there.
C Building is the area where prisoners who are being transferred from minimum- to medium-security cells are held, along with inmates who are being downgraded from maximum security to medium security, Coupe said.
Gov. John Carney said in a news conference he is praying for the fallen officer's family and praised the officers who "go to work every day knowing they may not go home, as do their families."
In a statement, the governor said the top priority is to determine what happened and how it unfolded over the course of the "long and agonizing situation." That includes holding those responsible accountable and making "whatever changes are necessary to ensure nothing like it ever happens again," he said.
Carney ordered the state's flags to be flown at half-staff.