Researchers haven't come to anything closely approximating a concensus about the circumstances of Tutankhamun's death. We know it was premature, as he was young, but that is the extent of our absolutes.
Several specialists have been circling about the notion that he was murdered, the most likely suspect being the cheif advisor, Ay, in an attempt to take the throne after the boy-king's death. X-rays of his skull reveal a calcified blood clot at it's base that could indicate a blow from a blunt implement, eventually resulting in death. This thickening of bone in the cranium would take a long while to build, indicating the young Pharaoh may have been bleeding a long while before he actually died, perhaps as long as two months. One investigator was quoted as saying "The blow was to a protected area at the back of the head, which you don't injur in an accident. Someone had to sneak up from behind.
Ay was a commoner, yet in the painting in Tutankhamun's burial chamber, the advisor is wearing the leopard skin garb of a high priest and the crown of a pharaoh during the "opening of the mouth" ceremony. Tut didn't have any children to succeed him, so it makes sense that Ay would be a likely candidate for taking the throne.
Add to this that there were a few suspicious deaths following soon after Tutankhamun's. His wife, Ankhesenamum, had the king of the Hittites send her one of his sons for a husband, as she did not wish to marry a servant such as Ay. A son was sent, but he was murdered before he even arrived.
Though it isn't certain, it seems Ankhesenamum eventually married Ay. There was a ring found with her cartouche inscribed next to his. Within three years of Ay's death, Ankhesenamum dissapeared. Ay himself died a few years after seizing the throne. His cartouches were eradicated, his tomb robbed and vandalized and his mummy vanished.
Another theory suggests Tutankhamun was murdered by General Horemheb, another commoner who became one of the king's closest advisors. After Tutankhamun and Ay died, Horemheb became pharaoh. It was he who erased the names of Ankhenaten, Tutankhamun and Ay removed from the royal list of pharaohs, suggesting he had personal investitures in the matter.He also took some of Tut's treasure and slapped his name on it.
For all we know, it was the king's personal attendant or cup-bearer, the only people allowed to enter his chamber or approach his back.
The study of the political structure suggests the king had few enemies. The was loved by the priests and the population, due to his re-establishment of the state religion after Akhenaton. In truth, there was little to be gained by either Ay or Horemheb with his death, as he was young and held very little, if any, actual authority.
On the pedestal of one of Horenhab's statues, there is a text leaving a message to all Egyptians, declaring he was loyal to his king and carried out all his orders faithfully. He also warned not to forget what foreigners did to King Tutankhamen.
There are other theories stating the mummy suffered the headwound long after death. The poor state of the body makes it difficult to discern.
Popular theory is that he was murdered. It may never be known for certain.