 What did the beings use to ensure they killed every human?
It was the hour of indecision when day has not quite made up its mind to leave and night has not yet attacked in force. A month later the meat animals, the birds and the household pets began dropping as rapidly as the flies which had dropped earlier. Within a year it was obvious to everyone that man was the only animal left on earth. The panic which had begun with the death of the animals was quieted somewhat by the fact that humans seemed immune to the pandemic. Then just one year ago, the first human became infected with the strange malady. Less than a month ago no more than a few thousand people remained in New York. Before the plague the Bureau of Vital Statistics had been one of man's crowning achievements.
 The Bureau's Index
 Charles's brain-waves  The Bureau's Index  A machine they brought from their home planet  Spies throughout the world