Terrorism and municipal water supplies

Has there ever been a recorded instance of terrorists (or anyone) poisoning a municipal water supply with the objective of killing people.

I’m surprised this hasn’t been done in some of the ethnic cleansing conflicts, but I’ver never heard of it happening. Is it harder to do than one might imagine or has it been done before?

This has come up before in various discussions on terrorism. I read in one site that some people tried pouring cyanide or arsenic into the water supply that was supposed to be used for some Nazi war criminals awaiting trial, but there was not enough of the material to do any harm.

A municipal water system is very big and would require a lot of nasty material to poison people.

The traditional method of poisoning water supplies involves the introduction of self-replicating toxins such as typhoid, dysentery or cholera. More information on historical usage may be found here.