Mason, neighbors said, lived just a few houses away at the corner of South Carey and Carroll streets in a basement apartment. He has prior addresses in Sykesville, court records show.
Evelyn Lawson, Mason's grandmother, said he is a churchgoing, married man and a good father to his 1-year-old daughter. She said she could not believe the allegations.
"This is all news to me about what happened," she said. "His family all loves him. We all love him. This is a terrible tragedy."
Police have not released specifics of the charges, and Mason does not have a lawyer listed in online court records on these allegations. Online court records show multiple arrests and assault and drug convictions.
Several residents knew Mason as "P.J.," and said he often parked his four vehicles — two Trans Ams, a Subaru Outback and a blue pickup truck — on the street next to his house, taking up every nearby space.
Henry Ott, 67, lived next door and said he often argued with Mason. The two had feuded because Ott kept watch of Mason's comings and goings.
"He tried to rule the neighborhood and tell the people how to live," Ott said.
Melissa Borck, who grew up in the neighborhood, said Mason could be standoffish. His treasured cars were "all he talked about," she said.
She said she was headed to the store at about 5 a.m. on Tuesday when she saw Mason sitting in one of his black sports cars across from his home. He motioned for her to come over, Brock said, but she refused.
"Happened right after that," she said. "I'm shocked. Maybe I'll cry later when I see it [on the news]. I'm still shocked."
As detectives overheard neighbors talk openly about Mason, police cordoned off his home and knocked on the door. No one answered, and police soon summoned a red tow truck with flames on the hood to tow away one of Mason's cars parked on Carroll Street.
In addition to Mason's arrest, police said they had charged a 16-year-old, Nazir Williams, in a fatal stabbing that occurred downtown after the Super Bowl parade in February.
Baltimore Sun reporter Kevin Rector contributed to this article.