PITTSBURGH—The death penalty trial of a man accused of having killed a western Pennsylvania police officer nearly three years ago has been delayed until next year.

The Allegheny County district attorney's office asked for the delay in the trial of 35-year-old Ronald Robinson, saying a key witness in the case cannot be located.

The trial is now scheduled to begin Jan. 28, but court-appointed defense attorney Veronica Brestensky says it could begin sooner if the witness is found.

Prosecutors allege that Robinson gunned down Penn Hills officer Michael Crawshaw shortly after killing 40-year-old Danyal Morton over a $500 drug debt in December 2009.

Deputy District Attorney Mark Tranquilli said he had been in "intermittent" contact with the unnamed witness, who he said had "expressed a fear of testifying."