The most embarrassing part of the bungled investigation was that the car's new owner found the weapon, prosecutor's spokesman Jann Scheerer admitted to the Weser Kurier newspaper.
The VW Golf belonged to a 35-year-old Iraqi man named Ali B. who reportedly shot his 13-year-old daughter Souzan in the head and neck in the town of Stolzenau in December 2011. The girl had apparently moved out of the family home because of tensions with her parents.
Ali B. disappeared immediately after the daylight killing, and his car was discovered a few days later in the town of Minden.
The gun was discovered in August, but prosecutors did not reveal where and how until Friday.
Despite the use of sniffer dogs, police were unable to find the gun in the car, and prosecutors then decided to sell the car for "economic reasons" – in other words, to save on the cost of storing it.
An international search for Ali B. is still ongoing.
Police are also searching for Souzan's mother Hazna K. who moved away from Nienburg with three sons, aged two, six and eight, after the killing. A recent search of her new apartment has shown that she and her children have now also disappeared.