Project Sign was the U.S. government's first 'official' investigation of the UFO phenomenon. The project began as a result of Gen. Nathan Twining's recommendation that the military investigate UFOs.
Behind the scenes, most people involved in the project favored the belief that UFOs were extraterrestrial in origin, the ETH (Extraterrestrial Hypothesis). A top-secret document titled "An Estimate Of The Situation" was drafted which reportedly stated that the although there was no proof, the belief that UFOs were extraterrestrial in origin may be the best or correct answer. All copies of the document were reportedly ordered to be destroyed and the project was officially ended without any official conclusion released.
Project Sign was then replaced with Project Grudge, a similar study of the UFO Phenomenon, this time staffed by people who openly opposed the notion that UFOs could be extraterrestrial craft. Not surprising, the conclusion reached by Project Grudge was that there was basically nothing to the UFO phenomenon.
Project Sign: UFO Study Group