We cannot say for sure what you may be dealing with. The trees may have declined due to poor site conditions, drought, poor drainage, possible insects, etc. You did not mention symptoms, how old the trees are, etc. Once a tree is dead it is difficult to diagnose. Here is our website on Dutch elm disease for more information https://extension.umd.edu/hgic/topics/dutch-elm-disease-trees
If you have photos of trees with symptoms on the foliage, bark, etc, please send so we can see what you may be referring to.