Investigators have “no idea” why a Maryland man fatally shot his parents at their home and gunned down two other people at a convenience store before setting fire to his apartment and killing himself, a police official said Monday. A gun that Joshua Green, 27, used in Sunday’s deadly shooting spree was registered to him and had been legally purchased in 2020, according to Baltimore County Police Col. Andre Davis. “As to why this occurred, we have no idea yet,” Davis said at a news conference. “Our homicide detectives are tirelessly investigating a motive, but it appears that the shooter acted alone.” Police believe Green killed his parents—Douglas Green, 58, and Olivia Green, 62—at their Baltimore County home before he drove over to a Royal Farms store in Essex just before 7 a.m. on Sunday. Green blocked in another car in the store’s parking lot, got out and shot and …