The man behind a shooting near the Mexican Day Parade route in Midtown Manhattan on Sunday, injuring a 17-year-old girl, could only muster up a “My bad” after officially being charged Monday.

Cops say 35-year-old Joel Nieves of the Bronx opened fire on 27th Street and Park Avenue at around 5:01 p.m. on Sept. 21, a minute after a 24-year-old man was stabbed on 28th Street and Madison Avenue.

According to sources familiar with the investigation, the stabbing occurred after two different groups — believed to be affiliated with gangs — got into some kind of argument that resulted in a 14-year-old brandishing a knife and stabbing the victim in the back.

After the melee, an eyewitness said she was walking to CVS with her dog when she spotted two men passing off a gun. “I was standing here, an

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