All the evidence - the victim's injuries along with the bloodstains and other evidence in the room - indicates an attack carried out by one individual wielding a small knife in his right hand. It happened quickly, but not every injury was inflicted at the same moment. It is likely he grabbed her hard by the elbows to restrain her as he worked his way into a secure position behind her. At that point he held the knife to the right side of her neck and inflicted a couple of relatively light wounds. She tried to escape, moving diagonally across the room toward the desk, and in the process of recovering his grasp, he pulled out some of her hair. Then he threw her to her knees, straddled her from behind, got a solid purchase on her face with his left hand, plunged the knife up to the hilt in her neck, and pulled with savage force, to inflict a gaping, three-inch-long wound. That is exactly consistent with the bloodstain patterns on the floor and the wardrobe.
Take a look at this:
http://www.injusticeinperugia.org/RonHendry100.html
I spent a lot of time working with this guy. He has based every point of his reconstruction on tangible evidence from the crime scene and autopsy photos.
I have looked at autopsy photos from other cases, showing the damage a single assailant can cause in a frenzied attack. No one could ever say (and at the trial, no one did say) that the injuries done to Meredith Kercher were inconsistent with an attack by one person.
But, at the same time, one can offer rank speculation that cannot be conclusively disproven. That is what Massei did in his motivation document.
