Merely placing the bag over one's head to suffocate isn't sufficient: The suicidal person has to take sedatives or use an asphyxiating gas.
I'm thinking that this since this was Brooklyn, they either didn't have a car or a garage to commit suicide by running the vehicle's motor in an enclosed space.
This is a false statement, neither sedative nor "asphyxiating gas" is necessary, other than the carbon dioxide that accumulates inside the bag.