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.
“I felt bad because he’d say, ‘Hi, Kerri,’ and I wouldn’t remember his name,” Ms. Owens said. “When I saw his picture on the news, my stomach dropped. I couldn’t believe it.”
Quiet as he appeared to be in person, he loudly spewed murderous hatred and bigotry online. For months online before the shooting, Mr. Bowers unleashed a torrent of anti-immigrant and anti-Jewish invective and conspiracy theories on Gab, a social media network that has been embraced by white nationalists. He had 21 guns registered in his name and carried out the massacre with an AR-15-style assault rifle and three handguns, the authorities have said.
Ms. Owens said she was stunned by all of this, echoing the shock of other neighbors who told the Pittsburgh Post-Gazette that Mr. Bowers had been a quiet neighbor and had lived with his maternal grandfather until the grandfather died in 2014.
The apartment building where Mr. Bowers had most recently lived is in Baldwin Borough, a hilly, middle-class Pittsburgh neighborhood that is a 30-minute drive south of the Tree of Life Congregation in Squirrel Hill.
His apartment is one of 11 in the grandly named McAnulty Acres, a two-story building that includes the offices of a plumbing company.
A rusty barbecue smoker and an empty beer bottle sat outside on Sunday.
Law enforcement officials had cordoned off the street on Saturday and robots entered Mr. Bowers’s apartment, said a neighbor, Jerry, who declined to provide his full name. On Sunday, there were no signs of police or F.B.I. activity.
Mr. Bowers seemed to leave for work for a few days at a time, but when he was home, neither Mr. Bowers or his teal-blue sedan moved, Ms. Owens said. Her fiancé, Chris Hall, 28, said he sometimes came home late from work to see Mr. Bowers sitting in his car, smoking and apparently listening to the radio.
“There was nothing about him, not even a bumper sticker on his car,” Mr. Hall said.
Ms. Owens would sometimes hear him through her wall, chatting with the landlord when he went to drop off his rent — in cash — at the landlord’s office around the corner.
But he had no guests. He lived alone. He watched television late into the night sometimes. He used a post office box instead of the mailboxes at the apartment complex. But nothing about his behavior concerned her, and their conversations never went beyond pleasantries, Ms. Owens said.
“I don’t know if he had any friends, anywhere,” Mr. Hall said.
She said she last saw his car when she went to work on Friday morning, but does not know whether he was still there when she returned that night. Like much about him, the memory is opaque.
“It’s very unsettling knowing all that stuff that was used to hurt those people was on the other side of the wall,” she said. “I didn’t see any signs. I can’t even comprehend that he had that much hate and seemed so normal.”
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Trip Gabriel reported from Pittsburgh, Jack Healy from Miami and Julie Turkewitz from Denver.