Dead trees possibly poisoned

My family owns a lake house in Michigan. A rich developer purchased the house next to our property with the intention to redevelop. He now owns all the land behind our house except for a narrow driveway that borders the edge of his property. Last summer, he wanted to cut down a series of eight trees that flank our driveway, but we reminded him that they are on our property and we don’t want to cut them down. He backed off. But this summer we came back to find that all eight trees he wanted to cut down are suddenly dead. I don’t have any evidence, but I’m suspicious that he poisoned them. What are the next steps I should take? Should I have soil samples taken? The trees are like 50 ft tall so it’ll be very expensive to have them removed.