 What did the beings use to ensure they killed every human?
A month later the meat animals, the birds and the household pets began dropping as rapidly as the flies which had dropped earlier. 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. Before the plague the Bureau of Vital Statistics had been one of man's crowning achievements. Housed as it was in a huge metallic globe of a building, it contained computers which kept exact account of every human on earth. Once a year the Bureau issued The Index, an exact accounting of Earth's four billion inhabitants.
 The Bureau's Index
 Charles's brain-waves  The Bureau's Index  A machine they brought from their home planet  Spies throughout the world