Those beholding the cloaked men doing this rush quickly to their kin and tell them of it. Local leaders in the communities along the rivers ban drinking from its waters, as for all they know it could be some sort of poison. Water will be imported from elsewhere to those communities along the river. Meanwhile, several squads of soldiers and a handful of Duneshifters are dispatched to investigate and, if possible, capture these strange men.
They manage to maim a few, but are outnumbered in enemy territory, and are quickly subdued. The Easterlings wished to take them alive, so they had to cut off several limbs in order to stop them.
Also, poisoning the Caspian won't do much. Unlike Russia, we don't drink salt water. Also, whatever the substance was, it would be far too diluted to have any effect.
The bodies are treated as if they are carriers of the plague. Plague doctors in their signature uniforms arrive to take the bodies away in wheelbarrows and burn them in a remote location in the desert. Anyone who had had contact with the people are placed under house arrest for two weeks enforced by guards at their doors.