Arias claims she did it in self-defense, and says she was a victim of physical, emotional and sexual abuse.
However, the prosecution is trying to prove Arias was a former lover out for revenge.
Maricopa County Medical Examiner Kevin Horn testified most of the day Tuesday, detailing the stab wounds Alexander suffered in the June 2008 attack.
Prosecutor Juan Martinez asked Horn to explain the knife wounds on Alexander's hands.
"I believe the wounds to the hands must have occurred before the fatal injuries of the head or of the throat," said Horne.
"So what you're saying, is at some point this individual grabbed the knife?" Martinez asked.
"Or attempted to," replied Horn.
Horn also described the slash wound to Alexander's throat, saying it was three to four inches deep and went to the spinal cord in the back of the neck.
"He has two major vessels that have been cut. He's going to lose a great deal of blood very quickly. He's going to lose consciousness, within seconds likely and die a few minutes later."
But the prosecution alleges when Alexander didn't die quick enough, Arias allegedly shot him in the head.
"Can you tell with regards to the gunshot wound to the right temple whether or not he was alive at that point?" asked Martinez.
"Again, there's a wound going through the head and I don't see hemorrhage in the brain. I can see a wound track through the brain. All I know is that there's a bullet going through the brain," said Horn.
Later, Horn clarified that there was no bleeding in the brain, which typically happens when the victim is alive.
Prosecutor Martinez pointed out that Alexander was possibly dead before being shot in the head.
Arias claims she was being attacked and was forced to kill.