DOVER, Del. — Delaware’s Supreme Court has denied an appeal by a convicted killer serving life in prison for fatally shooting a police officer.

In a ruling dated Friday, a three-judge panel upheld the conviction of Derrick Powell, who initially was sentenced to death for the 2009 killing of Georgetown police officer Chad Spicer. Powell was resentenced to life in prison after the state Supreme Court declared Delaware’s death penalty unconstitutional last year.

Powell’s attorneys argued that a judge erred last year in rejecting his motion for post-conviction relief. They claimed that trial prosecutors violated Powell’s rights in failing to disclose the existence of a witness who may have been helpful to the defense until lawyers had rested their cases and the jury was waiting to hear closing arguments.