Answer: cooperated

AnswerCrossword Clue
COOPERATEDTurned state's witness, say
COOPERATEDWorked together
COOPERATEDActed jointly.
COOPERATEDCaskmaker and a Kennedy worked together (10)
COOPERATEDTurned state's evidence
cooperatedAssist someone or comply with their requests
cooperatedCOOPERATE, to work together
cooperatedAct jointly; work toward the same end