5:12 PM: An associate of Heavy D's just told us the rapper had pneumonia.
4:28 PM: Law enforcement tell TMZ ... Heavy D had just returned from shopping and walked up some stairs when he began having trouble breathing. Adam Mills, a designer who was in the building and knew Heavy D, found him leaning against a railing and clearly in distress. Adam tells us he cradled Heavy D in his lap as others called 911. Adam says Heavy D was conscious and said to him, "I can't breathe, I can't breathe." When paramedics took Heavy D to the hospital he had a pulse. People in the building are saying he died of a heart attack, but we have not been able to confirm that. Law enforcement sources tell us ... no foul play was involved and so far they only know the cause of death was "medical."