PITTSBURGH — Robert Bowers, the man accused of gunning down 11 congregants in a virulently anti-Semitic rampage at a Pittsburgh synagogue, was an isolated, awkward man who lived alone and struggled with basic human interactions, neighbors and others who knew him said on Sunday.

“He was in his own little world,” said Jim Brinsky, 46, a friend from childhood.

Mr. Brinsky said he and Mr. Bowers, 46, grew up together but that he never met Mr. Bowers’s parents and got the impression that Mr. Bowers had a difficult home life. As teenagers, he said they built pipe bombs and blew up watermelons and trees as pranks.

They drifted apart by the time they got to Baldwin High School, where Mr. Brinsky said Mr. Bowers wore a camouflage jacket and drifted alone through the halls. Mr. Bowers is not listed in any activities or sports in his 1989 junior-class yearbook, and he does not appear in the next year’s book as a senior at all. It is unclear whether he graduated.

“He was pretty much a ghost,” Mr. Brinsky said.

His next-door neighbors also described him as a man who was barely there. In the shabby one-bedroom ground-floor apartment where he had lived for about a year and a half, Mr. Bowers made such an insubstantial impression on his next-door neighbor, Kerri Owens, that she forgot his name soon after he introduced himself. Mr. Bowers said he worked as a truck driver and needed the apartment primarily to store his stuff, Ms. Owens said.