STARKE, Fla. -- Danny Harold Rolling was put to death in Florida's death chamber Wednesday evening, 16 years after murdering five university students.

When asked if he wanted to make a final statement, Rolling turned his head, which was strapped down to a gurney, to look at the 47 witnesses in the execution chamber and began to sing.

Media witness Steven Stock reported Rolling sang a song with the chorus, "None greater than thee oh Lord," for about two minutes.

After two minutes, Rolling stopped singing and the audio system was turned off, according to Stock.

"We believe he started talking or singing again. His lips started moving, like was talking or singing. He continued to do that for a couple of minutes," Stock said. "He labored to breathe until about 6:11 p.m. and then his breathing stopped. Within a minute, one doctor came out and then a second doctor came out and he was officially pronounced dead at 6:13 p.m. by the warden."

After witnessing Wednesday's execution, Christa Hoyt's stepmother Diana described it as the final chapter of a book.

"This is our final chapter of this book. We're ready to start all over again," Hoyt said. "This is all about what happened to our children. This man brought this outcome to himself, and the law of the land carried through to show us justice."

Rolling, 52, had challenged the constitutionality of the chemicals used in Florida's execution procedure. The U.S. Supreme Court has turned down the same arguments in two other Florida executions this fall.

His five victims' bodies were found over a three-day period, some mutilated, posed and sexually assaulted.

Rolling had his last meal of lobster tail, butterfly shrimp, baked potato, strawberry cheesecake and sweet tea shortly before noon.

Death Row officials said Rolling was calm and cooperative. He spent several hours Wednesday with his brother Kevin, and his brother's pastor, Jim Wallingworth.

Rolling received a physical examination Wednesday afternoon. Shortly after being fed his last meal, Rolling was strapped on a gurney, placed on a heart monitor and wheeled into the execution chamber, where eight carefully marked syringes were waiting.

The first two syringes contained sodium pentothal to render him unconscious. The third, a saline solution, acted as a flushing agent. Another two containing pancuronium bromide paralyzed him. The final two doses of potassium chloride delivered the lethal doses that stop his heart from beating.

Before the execution, Hoyt said she would rather see Rolling get the electric chair like infamous serial killer Ted Bundy did when he was executed in Florida. Lethal injection became the alternative form of execution in 2000.

"I'm a nurse and I've seen patients die, and they died a much more horrific death than what this man suffered through, that's for sure. He relaxed, went to sleep, did not feel anything from what it looked like to me," Hoyt said shortly after witnessing Rolling's death.

Hoyt also commented on Rolling's final statement.

"I didn't appreciate his song. I didn't understand how he could sit there after the tremendous crimes he has committed and talk about angels watching over him. I don't understand that," she said.

State Attorney Bill Cervone said he saw no emotion on Rolling's face as the lethal injection process was carried out.

"As I watched what was happening, through my mind went the images of what he did to these victims. I saw the horror that he perpetrated on them physically. I know the emotional horror that he perpetrated on them. To watch his death in such an antiseptic and clinical environment convinces me that the punishment does not fit that crime," Cervone said. "We are, however, a society of laws and this is what the law said should be carried out. That's what happened this evening."