TRURO, N.S. - Terry Dean Allen was acquitted Monday of killing three-year-old Samantha Mercer over four years ago after a judge ruled that the Crown failed to prove that he caused the child's fatal head injuries.

Judge Alan T. Tufts said in provincial court that while he doesn't “accept or believe” all of Allen's testimony into the 2005 death, it is not the role of a trial judge to discover “what happened.”

He said his duty was to determine if the Crown proved beyond a reasonable doubt if Allen was responsible for the little girl's death.

“The Crown failed to do this,” Tufts said in a 61-page written decision.

“In the end, there may never be any clear answer as to precisely what occurred. The evidence supports more than one explanation.”

Allen, 28, of Truro was charged with manslaughter after Mercer died two days after she was injured in her Truro home on March 1, 2005.

The Crown alleged that Allen, who was living with the girl's mother at the time, injured the girl by deliberately slamming her head into a bedroom wall.

But Allen insisted the girl was hurt during a fall down a flight of stairs with a large dog - a claim that drew conflicting testimony during his trial earlier this year.

A mechanical engineer who was commissioned by the defence testified that it was possible to suffer that type of severe head injury in a fall. However, Chris Van Ee added that “data also show that it is rare.”

The child suffered massive brain damage, a fracture to her upper left arm, compression fractures to eight of 12 vertebrae in the middle of her spine, and bruising from head to toe during the incident.

She died after being taken off life support in a Halifax hospital.

Several medical experts, including Dr. Matthew Bowes, Nova Scotia's chief medical examiner, testified that children who fall down stairs generally do not suffer such serious injuries and rarely die from them.

“It is virtually unknown to have this severity of injury with a child falling down a stairs,” Bowes testified in June.

However, Ontario pathologist Dr. Michael Pollanen, told the court that the child could have sustained her injuries in either scenario.

Tufts said it is likely that Mercer was either thrown on her bed by Allen either playfully or in frustration, or she was bouncing on her bed, when her head struck the wall.

But the judge said he can't exclude the possibility that the girl did fall down the stairs as Allen claimed and suffered her serious injuries then.

“In my opinion the accused's testimony cannot be rejected,” the judge wrote. “His testimony raises a reasonable doubt ...”