Project Grudge replaced Project Sign in April 1949. In December 1949, a magazine article on UFOs written by the famous aviation writer, Donald Keyhoe, based on his private investigations and military contacts, elicited enormous media attention. In it, Keyhoe insisted that UFOs were alien spacecraft and that the U.S. Government was keeping this knowledge secret. In response to the furor that Keyhoe's article caused, and to demonstrate that there was nothing to get excited about, the Air Force reduced Project Grudge to a routine intelligence effort. However, in October 1951 , Project Grudge was returned to its original status as a special project. This investigation ended in March 1952. The final report suggested that most sightings had been explained. However, a large percentage of the reports were left either unexplained or only conditionally explained.