PITTSBURGH — Trial opened for one man as charges were dismissed against his co-defendant in the slayings of five people and an unborn baby at a western Pennsylvania cookout almost four years ago.

Allegheny County Common Pleas Judge Edward Borkowski on Monday granted a defense motion to dismiss the case against 31-year-old Robert Thomas in the March 2016 killings in Pittsburgh’s Wilkinsburg suburb. After the dismissal, trial opened against his 33-year-old co-defendant, Cheron Shelton.

Authorities had alleged that Thomas opened fire on one side and Shelton then gunned down victims running onto a porch for safety.

Thomas’ defense sought dismissal after prosecutors decided against putting a key witness on the stand. Defense attorneys said the witness had offered testimony in other cases, was alleged to have confessed to a role in a 2013 drive-by shooting that killed a 15-month-old boy, and had been offered relocation and financial assistance.