A number of private security researchers are increasingly voicing doubts that the hack of Sony‘s computer systems was the work of North Korea. President Obama and the F.B.I. last week accused North Korea of targeting Sony and pledged a “proportional response” just hours before North Korea’s Internet went dark without explanation. But security researchers remain skeptical, with some even likening the government’s claims to those of the Bush administration in the build-up to the Iraq war. Fueling their suspicions is the fact that the government based its findings, in large part, on evidence that it will not release, citing the...