What were Agent Dewey's two concepts regarding who murdered the Clutter family and which did he believe was true?

Concepts:

He still entertained a pair of opinions—or, to use his word, “concepts”—and, in reconstructing the crime, had developed both a “single-killer concept” and a “doublekiller concept.” In the former, the murderer was thought to be a friend of the family, or, at any rate, a man with more than casual knowledge of the house and its inhabitants—someone who knew that the doors were seldom locked, that Mr. Clutter slept alone in the master bedroom on the ground floor, that Mrs. Clutter and the children occupied separate bedrooms on the second floor.

Which did he believe was true?

Dewey—and the majority of his colleagues, as well— favored the second hypothesis, which in many essentials
followed the first, the important difference being that the killer was not alone but had an accomplice, who helped
subdue the family, tape, and tie them.