A man who spent nearly 25 years on Missouri's death row was executed Tuesday for the kidnapping, rape and stabbing death of a 15-year-old girl.

Roderick Nunley, 50, became the sixth death row inmate to be put to death in Missouri this year. During the execution, his breathing became labored for a few seconds. He briefly opened his mouth before becoming still.

He was pronounced dead at 9:09 p.m. CDT.

"Despite openly admitting his guilt to the court, it has taken 25 years to get him to the execution chamber," Missouri Attorney General Chris Koster said in a statement. "Nunley's case offers a textbook example showing why society is so frustrated with a system that has become too cumbersome."

Ann Harrison's disappearance and death haunted the Kansas City area in March 1989. She was waiting for a school bus on her driveway, 20 yards from her front door, when Nunley and Michael Taylor drove by in a stolen car and made the spur-of-the-moment decision to abduct her.

Her body was found in the trunk of the abandoned car three days later.

Both men were sentenced to death in 1991. Taylor was executed last year.

Of 20 executions nationally in 2015, all but four have been in Missouri and Texas. Missouri Gov. Jay Nixon on Tuesday denied a clemency request for Nunley, filed by death penalty opponents, asserting that racial bias played a role in the case because a prosecutor refused a plea deal that would have given Nunley life in prison without parole.

Nunley was black, as was Taylor, while the victim was white.

The U.S. Supreme Court, meanwhile, denied several appeals from Nunley's attorney, including one claiming that the death penalty amounts to cruel and unusual punishment.

Retired Kansas City detective Pete Edlund said the only thing cruel and unusual was how long Nunley and Taylor remained on death row.

"They just take forever to do the deed," Edlund told The Associated Press. "The delay in executing these two is just nuts because it didn't have anything to do with their guilt. It was legal mumbo jumbo nonsense."

According to prosecutors, Nunley and Taylor binged on cocaine and stole a car in the pre-dawn hours of March 22, 1989. At one point, a police officer from neighboring Lee's Summit chased the car but was called off by a supervisor when the stolen car crossed into Kansas City.

Later that morning, the men were driving around Kansas City when they saw Ann, her school books and flute on the ground beside her.

"They were just cruising and she's out at the driveway waiting for the school bus," Edlund said.

The girl's mother had stepped inside to get a younger daughter ready for school. When she heard the bus, she looked outside. The books and flute were still there, but Ann was gone.

"She knew something was wrong," Edlund said.

Taylor and Nunley had grabbed the girl and taken her to Nunley's mother's home. She was raped and sodomized, then stabbed repeatedly in the stomach and neck.

Taylor and Nunley put the girl's body in the trunk of the stolen car, then abandoned it in a residential area. The body was found three days later.

Edlund said the case was cracked months later when a man in jail for robbery -- and seeking a $10,000 reward in the case -- turned in Taylor and Nunley. Both men confessed, and some of Ann's hair was found in carpeting at the home where the crime occurred.

Edlund said Ann's father was a former reserve officer with the Police Department, and her uncle was a Kansas City officer.

"To all of us, she was part of our police family," Edlund said. "That made it even more important that we solve the case."