In a warehouse that Koenig leashed, investigators found a computer that he used to research information on how to kill a person with a single fatal blow to the head.
The couple has been married for 10 years, the report said.
Both reports said Koenig killed and mutilated Grace because she was preventing him from going to Thailand to enjoy a booze-filled holiday.
Koenig was arrested when he returned to Germany from Thailand, where he stayed for five weeks after killing Grace.
The International Business Times report said although Koenig received a life sentence, he would only serve 19 years due to a German law that says prisoners can be released at the age of 74. —KBK, GMA News