Ok, here's the scoop. Tut was around 18 or 19 when he died, and here are the suspects:
Aye- he was Akhenaten's visier (2nd in command) Aye was old. He would never be pharoah for the same reason as Horemheb. He ended up being Pharoah after Tut.
Horemheb- the general. He pretty much took care of most of the military
junk. He had a good excuse to kill Tut. If Tut had kids, then he would probbly never be Pharoah. But, if he eliminated Tut, then Aye would rule, and Aye was so old, that Horemheb would live to be Pharoah. If Tut had kids, then they'd have kids, and Horemheb could kiss the throne good-bye. He ended up being Pharoah after Aye.
Ankhsunamun- Tut's wife. Most of us eliminate her because she was so unwilling to marry Aye, and she ended up sending a letter to Egypt's mortal enemies, the Hittites, asking one of their princes to marry her instead. If she was so unwilling to marry Aye, why would she kill Tut in the first place? By the way, the Hittite prince died on the way.
For more info, go to www.google.com
and search. Also, there is a nice discussion in Queens of Egypt that talks about this.