In unusual murder trial, alleged foes are codefendants

As the carefree Caribbean carnival parade known as J’ouvert rolled down Blue Hill Avenue in Dorchester, Keith Williams and Wesson Colas allegedly confronted each other with guns in hand. Williams fired, but Colas did not, prosecutors said.

Caught in the middle on Aug. 23, 2014, was Dawnn Jaffier, 26, a bystander and youth mentor who was shot in the head after stepping out of the joyous procession. A second woman was shot in the leg but survived.

Three years later, Williams, 21, and Colas, 25, will sit just feet apart Monday in Suffolk Superior Court in a trial with an unusual legal twist: The adversaries are simultaneously facing first-degree murder charges, even though only Williams is accused of pulling the trigger.