They needed an explanation for why they closed the road. Canal Road in that stretch has no sidewalks or shoulders.
I am wondering where this woman who supposedly saw the violin case was walking as the primary walking trail there is the towpath which is across the canal from the road and several feet lower.
She might have been walking along the Capitol Crescent Trail, which parallels the towpath and which in places is alongside the road before it curves off and up the hill to Bethesda.