On the morning of 11 May, another PAVN assault was launched after being preceded by an artillery bombardment that fired over 8,300 shells into a defense perimeter that had shrunk to a mere by before the day was over. PAVN forces again forced their way into An Lộc, but the effort collapsed in the face of tremendous aerial attack, which cost the North Vietnamese 40 tanks and over 800 men. The reasons for the failure were not hard to discern. Beginning at 05:30 that morning and continuing for the next 25 hours, the U.S. Air Force delivered a B-52 strike every 55 minutes to support the defense. For the next three days, each time PAVN troops assembled to resume the attack, they were bombed in their assembly areas.