PORTLAND (AP) – State police detectives investigating the fatal shootings of two men on Maine’s online sex offender registry may never known precisely what motivated the Canadian accused in the killings, an official said.

To the disappointment of investigators, hard drives collected from computers used by 20-year-old Stephen Marshall in Nova Scotia have not yielded additional clues, said Stephen McCausland of the Maine Public Safety Department.

“There is more work to do, but so far we have not found out any answers to why he did what he did,” he said. “Unfortunately, we may never.”

Investigators have said Marshall looked up at least 34 names on Maine’s online sex offender registry before killing two men early on Easter.

Marshall fatally shot himself that evening when police stopped the bus he was riding on near Boston’s South Station, investigators said.

The laptop computer Marshall had with him when he died helped investigators in the five-hour gap between the killings of Joseph Gray, 57, at his home in Milo, and William Elliott, 24, who lived 30 miles away in Corinth.

But the hard drives from computers in his apartment didn’t yield useful information about the case, McCausland said.

Marshall, who lived in North Sydney, a town on Nova Scotia’s Cape Breton, was visiting his father in Houlton, Maine, when the killings took place. Detectives say he stole two handguns, a rifle and his father’s truck.

Marshall’s mother, Margaret Miles, told the Canadian Press that not finding out why her son acted as he did was “disappointing,” but she would try to remember him as “kindhearted” and “gentle.”

McCausland said it might have been too much to hope that investigators could explain why Marshall acted as he did.

“Sometime there are no rational answers to explain irrational acts,” he said.