ET explores this developing story...

TIM RUSSERT's personal physician, DR. MICHAEL A. NEWMAN, announced on MSNBC Friday night that an autopsy had been conducted and it was determined that the affable 58-year-old journalist had passed away due a heart attack resulting from asymptomatic coronary artery disease.

The doctor said that Russert's heart had been enlarged, and "fresh clots" were found in the coronary artery. Newman said that medical professionals were not able to resuscitate Russert following his collapse.