In 1856, Lt. G. K. Warren studied Great Plains wolf skulls and found that some of them were not full-blooded wolves. There have been many stories about violent hybrid wolf-dogs. It is possible that the Great Plains wolf's tameness and lack of fear of humans might be because they were mixed with domestic dogs. In North Dakota, by 1875 sightings of the wolf became rare, and by 1877 they were almost all gone. In North Dakota, two were sighted in 1915 by Remington Kellogg. The last known wolf was shot in 1922. The Great Plains wolf was officially declared extinct in 1926.