A Missouri inmate was executed Tuesday after spending nearly 25 years on death row for kidnapping, raping and fatally stabbing a 15-year-old girl in 1989. Roderick Nunley became the sixth inmate to be executed with lethal injection this year in the state.

Nunley’s execution was delayed by last-minute appeals from attorneys for death row inmates in Missouri, questioning the competence of the 50-year-old inmate’s lawyer. The Supreme Court, meanwhile, denied several appeals from Nunley’s attorney who called the death penalty cruel and an unusual punishment.

"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, after the execution, according to the Associated Press. "Nunley's case offers a textbook example showing why society is so frustrated with a system that has become too cumbersome."

Nunley reportedly kidnapped Ann Harrison in March 1989 while she was waiting for a school bus, about 20 yards from her house, in Kansas City area. Nunley was accompanied by Michael Taylor, both of who decided to abduct her when they drove by in a stolen car. Harrison's body was reportedly found in a trunk days after she went missing.

Nunley and Taylor were sentenced to death in 1991 and the latter was executed last year.

On Tuesday, Missouri Gov. Jay Nixon denied clemency request for Nunley. Death penalty opponents have argued that racial bias played a role in Nunley and Taylor's case as both of them where black, while the victim was white.

Nunley was given the lethal injection at 8:58 p.m. EDT and was pronounced dead after minutes.

Harrison's father, Robert, watched the execution along with the victim's uncle and two family friends.

This year, 20 death row inmates have been executed in the United States, 16 of which were in Missouri and Texas.