In the novel "The Adventures of Tom Sawyer" who commits the murder Tom and Huck witness in the graveyard?

Tom Sawyer and Huck Finn plan to go to the town graveyard at midnight to bury a dead cat. They come upon three men who are planning to steal a body from a grave. Hiding, Tom and Huck hear the men in a heated argument. The men are Dr. Robinson, Muff Potter and Injun Joe. A fight ensues. Muff Potter is knocked out then Injun Joe kills Dr. Robinson. He leaves with the evidence pointing to Muff Potter. The boys plan to keep silent. However, when Muff Potter's trial begin Tom testifies against Injun Joe. Potter is acquitted, but Injun Joe flees the courtroom through a window. Tom then fears for his life as Injun Joe is at large and can easily find him. You must read further to know how it all turns out.

This famous 1876 novel by Mark Twain (Samuel Clemens) contains many more interesting sub-plots about Tom and his friends and has been exciting reading for young people for more than 130 years.