The article from OP (which is from this year, rather than 2016 as the BBC article) says that they believe it came from the Mitanni, but that their conclusion is

The article from OP (which is from this year, rather than 2016 as the BBC article) says that they believe it came from the Mitanni, but that their conclusion isn't a definitive one: > The origin is another mystery around Tutankhamen’s blade. In the Amarna Letters, a diplomatic correspondence written on clay tablets and discovered in a royal archive in the south of Egypt, there is a list of gifts sent by King Tushratta of Mitanni in Anatolia, to Pharaoh Amenhotep III, Tutankhamun’s grandfather of Ancient Egypt when he married Princess Tadukhipa, the daughter of Tushratta. Among the gifts mentioned in the list is a dagger with an iron-made blade. > The research of Arai’s team states that this is very likely to be the same dagger for two main reasons. The first being that iron-processing technology was already common practice at the time in the Mitanni regions. And second, because the dagger’s gold hilt shows a low percentage of calcium with no sulfur. This feature indicates the use of lime plaster as an adhesive material for the decoration of this part of the object, a frequently used material in Mitanni, which was used in Egypt until several centuries later, during the Ptolemaic period. (I assume they meant 'which was [not] used in Egypt until several centuries later', otherwise that sentence doesn't quite make sense)