Andre Blake was convicted in the 2007 slaying of 16-year-old Kenneth Riddick of Cheswold. Prosecutors had alleged that Blake, who became angry at a woman who had been in a fight with his girlfriend, shot at her, but hit others, including Riddick.
Five people gave videotaped statements to police. All identified Blake as the shooter. The statements were shown to the jury.
Several witnesses were asked only brief questions before the statement was played.
The justices, in a ruling issued on Thursday, said the prosecutor should also have asked if those statements were truthful or not.
Information from: The News Journal of Wilmington, Del.