For further clarification. "Cool Mist" is not always the same as "Ultrasonic." Usually, a "cool mist" system is different than an ultrasonic system. In fact, I have seen only one humidifier that was an ultrasonic, but was called "cool mist." Of course, ultrasonics put out cool mist, but that is another story.
In short, make sure it is really an ultrasonic when buying a cool mist humidifier.
I also agree that if it was an ultrasonic humidifier it alone probably did not kill them. I would look into, however, what changes you made because of the humidifier, such as dampness, temperature, etc.
Chris D.