For what its worth---my opinion---a not very hard to prove terrorism dimension to this. I would be VERY surprised if it were an accident.
I have an exact opposite reaction. I would be very surprised if it were terrorism. Accidents happen at a far greater rate than terrorism. Terrorism generally has targets and weapons chose to give the biggest 'impact' such as a highly populated location or a location of vital interest, not a generic every day apartment in a generic every day part of NYC..