The problem is and will be hard to pinpoint and stop. Bayer 2-1 Systemic Rose and Flower Care, 3-in-1 Shrub Plant Starter, Complete Insect Killer for Soil and Turf, Fruit, Citrus, and Vegetable Insect Control, Seven Dust, Bug B Gone, Even the Provado Ultimate bug killer is being packaged with "free seeds for bees" making people think the product is "bee safe" at first glance. Nurseries and greenhouses treat with insecticides. Lowes, Home Depot, etc, etc, etc, etc.... Raising awareness is always a good thing, and it will stop some folks from using those things, but the choice usually comes down to their plants, flowers and gardens OR the bees.. Taking the time to figure out what is safe to use takes time and effort, compared to grabbing a bottle of the shelf and squirting it on.
Sending the bees in to be tested to find out what insecticide killed them is a complete waste of time. Even if you find out exactly there is nothing you can do about it legally. In this state if a commercial applicator sprays within a mile of registered hives you have a little recourse, but there is nothing to be done about a farmer spraying his own fields, or gardeners dusting their vegetables or apple trees etc..
Take a picture of the dead bees on the ground, and put it in the paper. Make sure to list facts and not gossip and hope for the best. As I said.. Most people will not care even a little. Not their problem. Their problem is keeping the bugs off their Tomatoes and cukes.