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

The justices heard arguments Wednesday in an appeal by 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.

Defense attorneys argue that a Superior Court judge erred last year in rejecting Powell’s motion for post-conviction relief. In that motion, Powell’s attorneys argued, among other things, that trial prosecutors violated his rights in failing to disclose the existence of a witness who may have been helpful to the defense until the state and defense had rested their cases and the jury was waiting to hear closing arguments.

According to court records, an eyewitness to the shooting, Damion Coleman, contacted police on Jan. 28, 2011, as the prosecution was still presenting its case. A prosecutor and detective interviewed Coleman two days later, and the detective wrote a police report on Feb. 2. But prosecutors did not tell the defense about Coleman until late on Feb. 4, after the presentation of testimony and evidence had ended.

Defense attorney Patrick Collins argued Wednesday that the deliberate delay by prosecutors left defense attorneys in an untenable position, even though the trial judge had offered them the opportunity of reopening the case.

“The timing of the disclosure put the defense in an impossible position,” Collins said, arguing that defense attorneys were focused on final jury instructions and preparing for closing arguments, and that they did not have adequate time to question Coleman and decide whether he was valuable enough to their case to reopen testimony.

“This wasn’t a slam-dunk witness who was a game changer,” Collins said. “This was a building block for reasonable doubt.”

Collins suggested in his written briefs that Coleman could have cast doubt on the credibility and testimony of another man who defense attorneys have suggested was the real killer.

Spicer was struck in the face by a single bullet as he sat in the passenger seat of a police cruiser following the pursuit of a car carrying Powell and two other men. The pursuit began after Powell shot at a drug dealer he was trying to rob in the parking lot of a fast food restaurant.

Defense attorneys suggested that Luis Flores, who was riding with Powell in the back seat of the car being pursued by police, fired the shot that killed Spicer. They noted that while both Powell and Flores had gunshot residue on their hands, Powell had no gunshot residue on his shirt and Flores was the major contributor of DNA found on the weapon’s trigger.

Deputy attorney general Kathryn Garrison, arguing for the state, said defense attorneys had ample time to question Coleman before deciding not to call him as a witness, and that the judge did not err in rejecting Powell’s post-conviction motion.

“Trial counsel, after interviewing this witness, felt that he would not be helpful to their case,” Garrison said.

A ruling by the Supreme Court is expected within 90 days.